The idea was to use the magic number as the source for cointype. If it's too big, as Tamas showed, perhaps a hash of it, and for coins without a magic number, a hash of their name (or some unique identifier).
That being said, I agree with Andreas that something that is 90% inter-operable seems very dangerous and will give people false expectations when they miss the corner cases. If the structure isn't going to be shared completely and have all support shared, having it completely incompatible along with a mechanism for converting part of it to another wallet seems superior. The worst types of losses will occur when someone tests out something with a limited use case, sees that it appears to work, makes dangerous assumptions, then gets burned when it's too late.
-Allen