Tom-
Have you tried the Therabite device? I'm not sure how it works if you have dentures, but there's alot of info on the site about it

http://www.oralcancerfoundation.org/dental/trismus.htm

It basically stretches your mouth open the same way a physical therapist does, except you buy the device and then you always have it with you so you can use it frequently throughout the day. I used to use mine all the time(a few minutes every few hours) but now I use it once in the morning and once at night. I'm not sure if it is still useful for you since it has been a few years since your surgery but it may be worth checking out.
Melissa