In Leshan, I didn't want casters wearing armor at all, after all they can easily learn armoring spells, and I shudder at the thought of 'tank mages.' (Of course, by the same virtue, we also did away with all the "flash bang boom" spells in our personal campaign, so the Mages have to think a bit, rather than just being mobile artillery.
The following is the solution as I printed it in my player handout for character generation. Bear in mind that we also made some modifications to armor and weapons, so the ap for the armors listed may not jive with those in the mongoose book, but it should give you the idea. . .
Spellweavers and Armor
Spellweavers require freedom of movement in order to weave the magical energies, and armor tends to interfere with that freedom of movement.
Non-metallic armors, if worn by a spellweaver, impart a penalty of –5% to all Spellweaving attempts per AP of the armor, and adds 1 combat action to all casting times.
This penalty is cumulative for every piece of armor worn.
Example: Pietor the Chicken Hearted decides that he should be wearing armor when he accompanies his friend Alexander the Mighty on an adventure.
Against his friends and tutors advice, Pietor outfits himself with a heavy leather helmet (AP2), a Cuirboulli Cuirass (AP2) and a pair of Cuirboulli Greaves (AP2).
These four pieces of armor (yes FOUR, each leg counts) give Pietor a penalty of –40% to all his Spellweaving Tests and every spell will take 4 combat actions longer to weave.
Metallic armors actually inhibit the flow of magical energies when attempting to weave a spell. Should metal armor be worn by a spellweaver, he gains the same penalties as listed for non-metallic armors, and, in addition, the cost of each spell is multiplied by the
highest AP of any metallic armor he is wearing.
Example: Pietor the Chicken Hearted, having barely survived an encounter with followers of the Bandit King, agrees to accompany the now recovered Alexander the Mighty in an attempt to recover Alexander’s favorite
horse, Pookums, whom he had to leave behind as he and Pietor beat a strategic retreat from the bandits after Pietor’s spells kept failing.
Deciding he needed more protection than he had before, Pietor borrows a scale hauberk (AP 4) a full Helm (AP 6) and a pair of Plate Greaves (AP 6). Pietor would now suffer a –110% penalty to his Spellweaving attempts, and every spell would take 4 combat actions longer and would cost
SIX times the normal POW to cast.
However, Pietor is spared from the embarrassment of such an attempt,
because Alexander took pity upon him and slew Pietor as soon as he waddled out of his house wearing the ridiculous outfit and decided to just go buy a new horse.
(Disclaimer: Your mileage may vary

)