Hey james, welcome to the site!
As Danny says, you've got a great surgeon, are six years along with no problems, I'd think you could do just about anything you want.
Check with Mr. Treacy's office, but I know there are several people on here who have gone back to competitive weight lifting, so lifting a moderate amount should be no problem.
Personally I'm not doing squats (no reason not to, just happen to do other things), but have for at least a year and a half been doing leg presses. I'm still keeping it very light (~250 lbs), with many reps. Upper body I've had no problem going back to the same amounts as before. I have no problem carrying 50 lb plates to the bench to press. I also do barbell workouts, have had no problems with up to 45 lb weights.
My own belief is that some weight lifting is necessary to strengthen the bones, so it should be just fine.
As to the failure / revision of an HR - there is a possibility of it, but for most of us it seems to be far off. The statistics look real good, but only go as far as 10 years now, with some people out to 20 years. This is a young technology, so we are the ones on the cutting edge.
I choose to believe that my two HRs will last forever. On my hips, that is. If I find out otherwise later, that's life in the fast lane. I plan to enjoy the hell out of them, and if they're revised, to enjoy the hell out of that.