I researched this too a while ago...
Not exactly what you want..and you may know this, but
They will let you send AC3 in 5.1 or whatever on the spdif output of a G5.
http://docs.info.apple.com/article.html?artnum=86438
explains it partially
A cheap decoder patched back in to the inputs of PTHD will kinda let you do what you want.
I would guess a rewire app/plugin/patch for this is also possible...or even likely
in a galaxy not too far away...
dos centavos
jtm