Wow, some success. Back at my parents' place for the weekend, I had access to a Windows machine. This allowed me to get the official Lego™ firmware onto the RCX, and then, well! The wonders one can work with known-good hardware.
The only Mac OS X 10.4.3 / USB Lego tower firmware downloader that worked for me is packaged with leJOS. Indeed, if I was more charitable I would try out their whole system, the core of which is an implementation of most of the interpreter-part of a JVM on the RCX, a nice piece of engineering. That's more than I hope I'll need, but we'll see.
The lejosfirmdl program happily downloaded the brickOS
kernel to the RCX for me. I have a minorly hacked version of it that I
can supply on request.
Once the brickOS firmware is on the RCX it doesn't take too kindly to attempts to replace it. Apparently one must ask it to commit hara-kiri, leaving only the Lego™ ROM intact and in charge. To quote the aging FAQ:
Because of a limitation in the Lego™ firmware,
firmdl3cannot delete brickOS. Instead, before downloading the new firmware, you must first stop the old brickOS program, and then hold down thePrgmbutton while hitting theOn/Offswitch. This will delete the old brickOS firmware, allowing a new download.
Next step: getting a brickOS program onto the RCX. I observe at this point that the linker and program downloader are fused together, and so I cannot just hope that NQC or leJOS will save my bacon this time.