Ok, I ditzed around with the new Mix_SetPostMix() function in SDL mixer 1.0.1.
I know I’m not doing things right (see the “NOTE:” comments in the source),
so please post some feedback and suggestions! 
ftp://ftp.sonic.net/pub/users/nbs/misc/mixer-postmix-test-0.0.0.tar.gz
Thanks!
-bill!