Thanks to iggy (Brian Jackson), a Gentoo dev, I was pointed to missing support for one of the disk controllers as one of the problems. Once I got that in, the master booted like a charm.
In the meanwhile, I discovered that newer versions of gpm fixed a problem that made a rescue attempt without /usr pretty much useless, because libgpm was in /usr and a ton of stuff linked against it via ncurses.
I also switched from raidtools to mdadm on claims that it’s newer, better supported and more consistent.
The LiveCD’s RAID and LVM autodetection seemed broken for me, because I had to do everything manually. Perhaps I’ll get a chance to look into that later.