Haven't crossed that bridge yet, but when I do I suspect I'll be using something very similar to the RQiii rules, ie Lunar Magic as sorcery applied to common magic, with the Lunar Magic skill taking the place of the manipulation skill and the Common Magic skill taking the place of the Grimoire.
Dark/dying moons mean no manipulation, crescent moons mean only one manipulation, half moons mean normal manipulation and full moons mean increased manipulation of some description. Perhaps the full moon would allow a Lunar Magician to substitute her Lore (Red Goddess) skill for her Lunar Magic (manipulation) skill or something generally beneficial.
Effects on non-progressive spells would need to be worked out but iirc Gods of Glorantha had some guidelines on that which might still apply.
Argh, you've whetted my appetite. Time to design a Lunar Magic user...