Everything was going swimmingly. I was booting OS X from my HP MIni's SSD. True, the screen was displaying at 640 x 480 resolution, and there was no sound. But it was working.
The next step was to install some kernel extensions to bring in the needed video, wifi, and audio functionality. Apparently I added one that was a problem, because the Mini suddenly stopped booting.
No problem! I could simply start the process over, reinstalling OS X from my 16GB SDHC card. Except that the card suddenly stopped working. Note to the world: Do not buy A-Data products. They are cheap, falsely advertised, and wholly unreliable.
I've since tried reformatting the card and I get a "read/write" error. It's toast after maybe one week and perhaps 5 major writes. Pure trash.
Today I'm off to Wal-Mart to get a working Sandisk replacement. With any luck, I will be able to report complete success with installing Leopard on my HP MIni in a few days.
After that, I'll put up a comprehensive guide, complete with links and warnings.