So there's apparently two paths to getting Build engine games to work in XP:
1. Follow an obtuse and capricious path to the letter, hoping and praying that you didn't mess anything up, or
2. Adjust the DOSBox settings accordingly:
Guess which one works. (On an Athlon 64 3500+ single-core, so YMMV)
Back to being a vengeful undead cowboy with a pitchfork, a sack-full of dynamite, and the complete scripts of the Evil Dead trilogy on tap for one-liners.