Thanks for your detailed explanation.
I had planed on writing a longer message to the list but, having just started a new gig, time is severely lacking. Also, it seems it'll be at least 2-3 weeks (thank Apple store ship times) before I get a chance to test anything on Mavericks. Here's what I can tell you so far:
* Replacing the GC is semi-problematic
But that also should not be a problem for Mavericks, because clearly GC works just fine in Mavericks.
Once that is done, I’d be happy to try pitching in to fix the method definition problem that is plaguing MacRuby on Mavericks.