For some reason I was under the impression that the latest Amaya version (v9.54) used Cocoa, and not wxWindows, which could be the reason why I didn't find the source for it (I knew where the gtk version was).
There's also Dillo, which is not really in the same level as all the other browsers, but is pretty neat. Last time I check they were moving from GTK+ to FLTK. Seeing as their engine is pretty small it wouldn't be as hard as porting something like Gecko or KHTML. The browser itself also compiles to a really small binary (350k I think). It's main problem is that it still doesn't handle CSS and other features that are considered basic.