I do agree that fighters with epic spells is annoying, but the sequencer robes themselves are a great idea.
To force mages to wear them to keep their spell stored would mean having to improve the AC of the robes considerably to make them worthwhile in a fight.
Would it be possible to take into account the wearers spellcraft skill instead?
for example, cantrips - lvl 2 spells = no spellcraft skill needed. Lvl 3 - 5 = 15 skill points needed... etc etc
so if a fighter wanted an epic summon then then would need to max out spellcraft and maybe skill focus in it as well? You may get some that would nerf their build enough to accomodate getting epic spells, but not many. but you would leave the robes open for lower level buffing spells that I assume you made them for.