Adding a LPD server with communication to a serial printer
should be easy.  However, you want a button on the printer
to initiate communication back to the serial port - that is
a lot harder.

What is coming up with the new username and password?
Can it be run on the point of sale terminal instead of the
m0n0wall box?  If the customer is being charged, then the 
cashier has to enter a sale for this anyway.  Why not have 
the point of sale print the username and password on the 
receipt and then automatically communicate that somehow 
to the m0n0wall box?  Alternatively, can the POS request the 
username and password from the m0n0wall box when printing the 
receipt for the sale?  It could all be triggered automatically
by selling the appropriate item, so the cashier would not
even have to hit a button on another unit.  You then also
don't have to maintain another printer or walk over to a
special box if they have POS terminals in multiple locations.

---Hillel