I don't see the tying of magic to properties as being insane, at least not reasonably static ones. Bloodline have been mentioned, date of birth hasn't but would be another possibility. Attributes are messy in that they can change over the character's lifetime (if you've got magic that only works if the strength is low, you may want to consider going for a pen pusher job rather than a mining one...[I curse thee with the spell of average strength! Rather interesting way of defeating an opponent...]), but anything that can be statically applied to character according to apparent logic or even hidden logic (such as those born on even minutes or hours get magic, which I'd expect would seem random unless it's been studied very carefully, and using accurate clocks which might not even exist), or even "truly" random can work. Ties are intended to be exported to the myth editor, so when fully fleshed out, any logic the myth generator can use ought to be exposed to the editor (and it's not impossible there are some variants the myth generator is blocked from using because the results are too chaotic or unbalanced, but are logically possible in the systems themselves).
Thus, what's available to specify with the fully developed myth editor ought to be what the systems used by the myth generator supports, which depends on what Toady and ThreeToe have both thought out and implemented ("thought out" doesn't necessarily mean they're original thoughts, but even if well used tropes are used, they still have to be turned into a logic system for the game).