Not sure if you're close to PA but Eurosports in Coopersburg has a great engine tech.. He's done a lot of internal work on these motors..
They are about 30mins from the NJ line off of rt78.. http://www.eurosports.net
I wonder if the light goes off for either pump? I was thinking one could exchange the sending unit for a real oil pressure gauge and fire her up quick and short to see if it has pressure if so I would wonder if you lost the drive/chain on the scavenger pump. I don't know but thought I would toss some ideas around.
Ahh yes! Back in the day you simply opened the oil cap and looked in to see if your Triumph was pumping oil properly. A solid stream on start up and then intermittent as the larger scavenge side emptied the sump.