This sounds a bit like the approach taken by wxWindows. It interfaces to GTK, motif, the Windows API, and OS X (I think). Anything that one or the other can't do is implemented directly by wxWindows. So everybody gets a high-functioning almost native toolkit. Pretty sweet.

Too bad I haven't been able to get wxPerl to compile, but that's just me :-)

I just realized that I was using the same sig for nearly three years.