I started this before Christmas, but only just got round to finishing it off...
The rules for Runemagic in MRQ are very customisable to give the flavour of magic you prefer, although this can be slightly obscured if you take them to literally - as written they suggest a platform-style computer game, where mysterious blobs of energy appear almost at random, and if you pick one up it grants you unusual powers. However, I don't find that approach very interesting or Gloranthan, so lets look at some alternatives.
Runes as Powered Crystals
Runes are described as being formed from the spilt blood of the Gods, and they require users to attune them before they can be used, which makes them sound like powered crystals from previous editions of RuneQuest*. In order to use them like this, and to return to the previously observed Gloranthan truth that everyone is capable of learning magic I'd propose the following:-
All Characters can learn Runecasting skills from their Cults/Schools/Guilds/Other Organisations (hereafter "Cults") and use these to cast (MRQ) Runespells whether or not they posess a Rune Crystal. They will, however require a focus for each spell. This may be a tattoo, an amulet, a medicine bundle, a wand, or an inscription on an appropriate item
Each Cult should have a list of which Runes it can teach RUnecasting for** (There may be two rates - one for the Runes associated with the Cult, and one for other runes they can teach for a price) and also a list of Runes they are forbidden to learn (eg Darkness runes for Yelmalions, or Death for Chalana Arroy).
If a character finds and attunes a Rune Crystal they get the benefit of integration listed in the Rules, and aso a bonus to the associated Runecasting skill (This might vary from crystal to crystal - a small, weak crystal giving only +5 or +10, a powerful crystal giving
+100% (So you only fail to cast the spell on a 96+) - It is generally not possible to tell what the power of a crystal is without attuning it first).
A Rune Crystal can only be attuned by one character at a time, and the crystal must remain in the characters posession to provide the bonus.
A character can only attune a limited number of crystals*** (and possibly only one of any particular Rune at once?) I'd also forbid the attuning of opposed Runes (eg Movement and Stasis, Life & Death etc) unless the character belongs to a Cult that embodies both runes.
Runes as a harvestable source of Energy
You could also treat Runes as a harvestable source of magic, a little like Ars Magica Vis, Under this paradigm, certain places are known to generate Runes of a particular type on a regular basis. Most of these will be under the control of Cults or Schools - They will often be shrines or temples and will, for example, produce the Runes on the cults Holy or High Holy days. This means that characters joining a Cult (probably at the initiate or equivalent level - Lay members are generally not committed enough) can be provided with one or more runes automatically, giving characters access to magic without requiring large amounts of "free" runes to be floating around.
Under this model, Godlearner Sorcerors and their schools will be happy (keen even) to trade Runes they can readily obtain for those that others have to spare. Traditonal theists, however will consider their runes to be more of a holy relic, and be less keen to swap and change...
Worship as a source of Rune Power
A slight varient of the above, working in a similar way, but without the actual Runes.
Voiroff suggested that something like this was in an earlier draft of Gods of Glorantha.
This is my take on it...
Characters who join a Cult at the initiate (or equivalent) or above level are automatically attuned to the Rune(s) of that cult for purposes of spell casting. (They do not get the benefits of integration unless they actually obtain an instance of the Rune). Obviously they will lose this attunement if they leave the cult, unless they have also got the benefit from another source (either membership of another cult with the Rune, possession of the rune, or Nysalor Illunination****)
Relinquishing Runes
Some of the following discussion is based on suggestions and conversation both on and off the playtest list following Steve Perrin's original draft of the Runemagic rules...
The one problem with using Runes to provide magic, especially in a "magic rich" world like Glorantha is that if all the characters opponents have Runes, and the characters are able to defeat them, then at the end of the adventure, the characters will all have more runes than they started with, thus being more powerful. This means their next opponents will need to be more powerful, which means more runes, which means, when they are defeated the characters will start to experience an exponential growth in Power.
Additionally, in RQ, traditionally one has often been encouraged to ransom your enemies rather than killing them outright - but if the only way to get their integrated runes is to kill them then carrying runes is equivalent to signing your own death warrant...
This leads to a couple of thoughts
Obviously a character can choose to relinquish a Rune - undoing the effects of integration - This may cost POW and/or require a roll
Maybe if you seperate a character from the physical representation of the Rune they have integrated they lose their connection to it (Maybe not immediately or automatically)
Killing a character may not immediately return the power to the Rune object.
Maybe when a character dies with runes integrated those runes dissipate, so characters don't automatically gain all the runes their opponent had if they kill them
Maybe the dissipated runes reappear where they were first found (Triggering a "Rune-rush" when the owner of a famous and powerful rune dies)
Footnotes
* RQII/III had two types of magical crystal, formed from the blood of Gods. Power Storage crystals, which one could use to hold a certain number of Magic Points above and beyond your own POW, and Powered Crystals which required attuning but then provided a specific
benefit, for instance doubling the effective MP used if the spell were resisted.
** I'd also rework the spell lists here, both to realign spells back to their appropriate (Gloranthan) Rune, and also, possibly, to provide some spells through more than one Rune
*** probably linked to POW?
**** An Illuminate is able to leave cults without fear of the spirits of reprisal, so I assume they can also retain their runic affiliations even if they would appear contradictory...