Yes, IMO a VERY important question that hasn't been answered yet.
I would be concerned that Google felt it was TOO open with Android proper and tried to restrict Wear even more.
If the current preview emulator is a good indication, then Wear is just another variant of Android. Many normal apps can run on it right now, though with issues. I tested my own app and it more or less worked, with a squished UI.
So I think a LOT of the Wear code is effectively already IN AOSP already.
But will Google release enough Wear specific code for custom ROMs to be built ? I very much hope so.