Wanted to report (mostly) success. iRiver H320. Intel MacBook OS 10.4.11.
First tried the RockBox utility (rbutilqt-v1.0.6b.dmg). Pointed it at a v1.30(eu) iRiver original firmware, it asked me to flash that, did that; let it load all the rest of the Rockbox software. Two problems: (1) wouldn't boot at all unless the charger was plugged in (2) it only booted the iRiver OS.
It wasn't clear to me whether the RockBox utility on the Mac was patching the firmware file (the h300.hex file). So I went to a WinXP machine, got another copy of the original firmware, then used fwpatcher.exe to patch it, and flashed that. After that, everything worked fine

If the utilty does indeed patch the firmware before telling you to flash it, then maybe my difficulty was that sometimes the H320 needs to be flashed more than once before it takes.
Anyway, thanks for all the wonderful and impressive work!!!