I know it is currently not possible to register typedefs for non-primitive types. Would it be possible to add support for this, given recent updates to the type info code?

It would definitely be possible to add this support. It was possible earlier too, I just haven't had the reason for doing it. In fact I've often thought about retiring the typedef feature entirely (don't worry, if that happens one day it will only be when I release AngelScript 3.x).