We have a crosshair overlay for that. Aim with the mouse and click to shoot, same how we emulate Namco Time Crisis for example. *edit* Oh, lightgun + layout instead of bitmap/screen might be a problem but nothing we can't fix or workaround. I don't know if it's currently possible MAME can know if the mousecursor is above a red LED in a layout.
Sean ok got it =) BTW I got a cheap TI Spelling Bee on eBay, the somewhat rare 2nd version, should be on your way soon.