Is there a simple way to setup ndps printers to allow both internal network
and external network access? (i.e. external access for mobile users).

Besides setting up two separate printers, one for internal access with the
private ip address and one for external access for the public ip address.

After looking at the printer setup in iprint, it states DNS or ip address of
printer. (So this would list the private ip address of the printer and not
the public one for mobile users).