Hi Fergal,
Yes, is the short answer, although it would probably be significantly easier (and cheaper!) to simulate one in software on a second monitor / PC.
I'm not an avionics engineer or anything but a bit of work with Google found this:
http://www.beckerusa.com/pdf/ - the IN3300 is a VOR/LOC indicator, download the PDF
IN3300-I+O , it's got wiring diagrams and everything. It seems that if you just feed +/- 150 mV into the correct pins, you'll get the appropriate indications on the display.
Apparently these things are governed by the
"ARINC standards", but they're not freely available - several thousand US dollars each!
Hope this helps,
cbl.