SCO Unix run interactive script when sending a print job

So far as I know (as a regular user of SCO Unix) the "@" and "%" prefixes have no meaning in SCO Unix and are probably something used by the ERP system. You can list printers using the command lpstat -pDl. If, as I suspect, you see lp5 and not %lp5 that would confirm that the prefix is something used by the application.

I believe the printer interface scripts are expected to work in the background without any connection to a specific interactive session - so they might not be a suitable place to introduce an interactive dialogue with a user.

If the application invokes lp or lpr - you could probably replace those with a suitable shell script.