I recently got a comment from Luke J. Crook that I should check out, among others, lispbuilder-sdl. Well, I just did, and it seems to work great on my mac (some notes further down).
Lispbuilder-sdl seems to cover pretty much everything I need from SDL, except for the OpenGL stuff. But I will try and see if I can not just use my own OpenGL binding in conjunction with lispbuilder-sdl. My guess is that this should work just fine. If that's the case, then my cffi-sdl project becomes quite pointless, and I'll just offer the lispbuilder guys a copy of my opengl binding for them to include.
A note if you're trying to install lispbuilder-sdl on your mac: I had to modify two things first. One is the cocoa-helper code which didn't find the SDL header on the system. That required a small fix in both the Makefile and the source code. Second was the loading of SDL by CFFI, which couldn't find the library. This is because I installed SDL through Fink, which places it under /sw/lib/. So I added that to the lookup path in the right way, and then everything was just fine.
So once again it seems that most of my work was quite spurious, but hey, live and learn. :-)