Keep an eye on eBay, cards aren't all that scarce, but they're pricey. It's the breakout box that's impossible to find, but I wouldn't be surprised if you had that for an early PCI version.
My DA playtime is in
Sound Tools on DigiDesign's
Sound Accelerator II. I used it to wring stereo out of the DuoDock, which had left and right channels fused into mono. The only editing I did then was a left/right/left/right track test file. I've also got PAS16 for NuBus and LC PDS.
That second link is for those with interests in the very beginnings of DA tools for the Mac.
Back on topic: Do we still have a topic?
There's precedent for gaining access to that disappearing half Meg of RAM. Connectix developed
Compact Virtual to get around such ROM based memory addressing limitations. At the dawn time NuBus architecture PMMU operations were moved on die for the 68030 and Accelerators for the 4MB limited 68000 Compacts became available. Some were equipped RAM SIMM slots implementing Silicon Disk/Virtual Memory capabilities up to 8x the MoBo's 4 on the floor.
The effort needed to develop such capabilities for the 33% increase available on the floor of the PowerMacs G4 under OS9 may or may not be a worthwhile endeavor. But doing it would certainly be cool as hell!
Back off topic: Not sure how far back you folks and your interests go, I'm new here so I dunno. If such historical tidbits about DA in the dark ages are off base, let me know.