To summarize the underlying problem, babylon5's Linux install is ancient and clogged with cruft, and its SCSI hard disks are dying rapidly.
Simple solution: Install an inexpensive SATA controller and a new mirrored pair of SATA disks.
Well, by swapping controllers around so that the existing SCSI disks aren't the first devices, I can get the machine — which is built on an Asus A7V333 motherboard — to boot from a SCSI CDROM which it will not, as a general matter of course, condescend to boot from. From that CDROM I can boot a Gentoo liveCD, set up the new disk, install Gentoo, build a kernel, set up grub, etc, etc, etc, all with relatively little issue (aside from the part where I typo'd "-mtune=athlon-xp" as "-mtune=athlonxp" which resulted in gcc declining to compile anything).
There's just one problem. Try as I might, this motherboard WILL NOT boot from SATA. Leaving me squarely behind the eight ball for upgrading the machine, I think, unless I hold my nose and suffer with PATA. It'd be cheaper to buy a new motherboard, CPU and RAM than to buy a pair of new SCSI disks.
(Update 2355: I didn't even have much luck with PATA so far. I even tried dropping in my one good spare PATA disk, an 80GB Seagate Barracuda of uncertain age, just to put a bootloader on its MBR to boot the SATA disk. No joy; grub would install but wouldn't load.)
Re: SCSI/SATA boot order
Grub probes the BIOS to map devices, yes. I've been thinking about the problem overnight and I think that with any luck, I MAY have a solution. Among other things, I've realized that part of my problem is grub doesn't understand JFS yet, so I'm going to be forced (for the first time ever) to use a separate ext3 /boot partition. I also may have forgotten to mark my boot partition on the SATA disk as active.
So, I'm going to try it all again. But not today. Especially since I CANNOT make the machine boot from the SCSI CDROM if there is a working and bootable SCSI disk on the same controller and it's one of the first two disks the BIOS sees. Oddly, if none of the bootable SCSI disks is among the first two visible devices, it's quite happy to boot from the SCSI CDROM.
Well ... OK, maybe today. Because another SCSI disk just failed, and I no longer have a boot mirror. So now I'm under time pressure before the remaining disk (which is actually the oldest) fails too.