Hi again Steve - I have had a good look at my injector pump, and it does not have a dipstick. There is however a red plastic "Oel" fill cap beside the thermostat/air filter housing. I did check my service records, and my mechanic did install a new thermostat in October; I didn't see the one that they put in. If necessary, I could e-mail you a picture of the injection pump and the housing in question.
I think that my mechanic feels that the housing is leaking as you say. I'd like to give this a try if possible. When this was not a problem, the engine ran beautifully (~1 year); I'd prefer not to yank out the injector pump until I've exhausted all other options.