Ever tried virtualisation?
VMware did a pretty good job at running 32 bit OS like WinXP in a window, while passing through USB hardware from the host OS (win10 64bit).
So if mach3 is running on a decent USB-PPT interface, i'd check that too.
I have run my car's OBD stonage interface via USBCOM serial port-USB bridge controller under such configurations successfully.
Well worth a try, imo. And oracle's virtualbox should do it sufficiently, too (for free).
EDIT: I googled "usb parallel port mach 3" and judging by the first three results it shouldn't be much of a problem to get things running that way.
The problem is latency.
CNC needs really precise timing, like realtime kernel precise, which is why the parallel port is on the PCI bus, USB adapters are a recipe for disaster.
I know you googled, but those hits are referring to purpose built USB input motion controllers that deal with timing internally. This thing isn't worth converting to a new motion controller...though somehow it is worth days of my time...