You need to trick the application in thinking there is a LPT1 port
- The printer that is on USB you need to share it. Call it "printer".
- Start DOS
Prior to running old DOS application type following:
net use lpt1: \\127.0.0.1\printer /Persistenst:Yes
Start old DOS application
This will redirect the LPT1 port to the share and print to the USB printer. You can put this in a script to prior to starting the application.