SDL_image 1.2.7 is the latest version that seems to work with the level and score readouts. Due to the various attempts to diagnose this flaw, the source can now be reverted back to the earlier code and, with SDL_SetGammaRamp commented out, we're good for Lion and Mountain Lion. Thanks to the AROS port, the confirmation that the main code was fine was enough to direct the efforts to understand the issue.
Hopefully I didn't screw this up. The sourceforge project administrator has ignored requests to include this source on his pages, and I've had requests for a more accessible source release. Here it is. Note that this merge is different to the last released source, largely to workaround graphics corruption on OS X Lion and later (due to something odd happening in the calls to SDL_SetGammaRamp. The game still runs rather slowly; I have run Measurements on it and the delay seems to be deep in OS X's image handling routines.