Or even just a compiler to C piggybacking off <objc/runtime/objc.h>; I think Apple still spends a lot of time making even dynamic class definition work fast. I haven't touched Cocoa/Foundation in a while, but I think (emphasis on think) a lot of proxy patterns in Apple frameworks still need this functionality.

there was MacRuby[0] which I seem to remember had an AoT compiler and was built on the ObjC foundation but was later abandoned.

[0] https://en.wikipedia.org/wiki/MacRuby

It was basically forked into RubyMotion, which is closed source but actively developed.