Rui CarmoHP, after a good six months or so, finally published version 6.3.4 of its PSC 950 drivers, therefore alleviating the need for my previous fix to get it to print on Panther.
Printing works OK (only a couple of basic tests so far), and the HP Director and HP Gallery were given a sizeable facelift. However, they still feel like they were designed in the 1980s - very slow, heavy (even ponderous) applications with poor Mac OS X integration. HP Director also has the nerve of setting itself up as a startup item (and taking its own damn good time starting up) without asking permission.
But, most important of all, I can use the scanner again. Until the next Mac OS X update, perhaps...
Update: one person (Hi Clint!) just sent me e-mail mentioning that using these drivers, the printer stops responding if you use Fast User Switching and switch to another account. I have not tried it myself yet, but uninstalling and reinstalling does not fix the problem - but then, why should Fast User Switching mess up printing? Oh well. Another six months for a fix, I guess... This hint seems to help, though.
SSL Woes
A word of warning to anyone trying to use the CA.pl utility on Mac OS X: the built-in Panther OpenSSL does not support the -selfsign option, which was introduced here. You'd expect Apple to have an update by now, or the OpenSSL guys to have better documentation...