Did you know that at the time development of JavaScript JIT for OWB was started there already existed a working MacroAssembler for JSC? (I had ported it from TenFourFox to WebKit.)
However I did never try to get the JavaScript JIT working (backporting of whole WebKit is keeping me busy enough) using that assembler but YarrJIT (regular expressions) is working well using it.
Unfortunately I didn't know of a way to get in contact with bigfoot, until now - I'd really like to port this to Leopard WebKit (mainly targetted at PowerPC OS X 10.5).

Please keep me updated in the progress of this - I'm also willing to cooperate in order to get this fully working in the not too distant future.