Conclusions:

With relatively little work, and very little subsequent administration (the X terminals seldom crash, and don't need software updates), we converted a bunch of "junker" PCs into rather useful X terminals (I've even done a 386 this way and it runs fine).

As a final note, it's important to point out the while the particular systems I've described here are PCs running Debian Linux, the same technique applies equally well to resuscitation of any older X11-capable machines, such as older Suns (Sparc ELCs), HPs, and SGIs...