This might be a little more work than you want but build your own? It looks like there are many technical issues like not being able to precisely control the length of pulses etc... which means you're kinda stuck with the whole USB -> non printer parallel port idea.

The only time I've seen a USB -> parallel port interface dongle that wasn't for a printer was with the old Cypress PSoC parallel port ICE. It didn't emulate a printer however and you downloaded firmware into it to make it run.