Gimp does this by including an OSX GTK theme as part of Gimp.app. I tried this same theme on Inkscape when first creating an OSX app of Inkscape and found it didn't work very well due to us using some different widgets or widgets of different sizes. Thus I decided not to include the theme as part of our official packages.
This could be revisited... Probably this would just need someone to update the theme in some cases to work specifically with Inkscape.
I'm afraid I've never played with GTK themes and am not near my wife's Mac at the moment. Does this involve editing bitmaps to the right size, or editing text files, or source code and compiling or what?
I don't have any devel stuff on the Mac, but if it's bitmaps or text files, I may be able to work with it.
Thanks,