MAME supports multiple (emulated) monitors, so the built-in serial terminal can coexist with your board's graphics display. If you use -numscreens 2 they'll appear in separate windows, otherwise MAME will put them side-by-side in a double-wide window.

For the hookup, you'll need to add an rs232 device, route the necessary signals between it and the UART device, and specify "terminal" for the last parameter. Look at the ht68k.cpp driver for a minimal working terminal hookup to an MC68681 (which for hookup purposes is the same as the SCN2681).