Man, the only way I can think of to have the data available from within a payment processor is to set local variables for the data you need when initiating a payment (open payment screen, quick pay, etc.)
There’s Data.Get() and also a GQL query getLocalSetting that might work. But to use the latter requires an API licence. I’m also not sure if the local setting retrived is local to the terminal or the message server.
Have a peek at this old thread for some ideas:
Payment processing is a complex beast; I’ve all but given up on it.