Tovec
Explorer
How 'bout...just going the "old school" route?
Under the Staff of the Magi, it will say, quite clearly, "Mages only."
Under BARD it reads, "The due to their extensive knowledge in legends and lore coupled with their spell-casting capacity and magical know how, the Bard is the only class with access to all types of magical items, as well as enchanted armor or weapons with which they have proficiency."
And each table can figure out which way the wind [of the Staff of the Magi] blows on their own.
Because many groups (especially newer groups/players) are going to now fight over the issue that bards should be allowed to use SotM because they have this line, whereas others are going to point to the staff to where it says "mage only" and say they can't. And it is an issue based solely on the words provided not on the interpretation you are trying to evoke. Unless the bard description has "mage" in it you are going to have this issue as people have become more used to keywords and "specific overrides general" so that specific of SotM will override general "bards can use all magics" even when the group itself hasn't decided (or worse when they have decided but now the rules are getting in the way).
The "old school" route may work for old schoolers who are used to it, but mid-schoolers and new-schoolers get hosed/confused?