Have you tested in previous Linux? I had one F103 project running USB CDC, which works in Ubuntu 15.04 or newer. But it doesn't work well in 12.04/14.04 and OpenWRT.

Wish you could do the same for a parallel port. :)
My CNC machine relies on 2 parallel ports and its becoming increasingly hard to find boards that have them. I should really just bite the bullet and upgrade the whole system.

Definitely, usb2lpt converters are exist. But they are not standard. If your software using default lpt io ports, it won`t work. So yea, upgrade might be the only one choice.