hmm... I'm in BC, I've had lots of bloodwork done to monitor my hormone and thyroid levels because we thought there was something wrong with me LOL I've never had to pay. I had OHIP while living in Toronto for about 4 years...i'm pretty sure i had bloodwork done and dind't have to pay either (although it was a while ago, i don't even remember what it was for).
I dunno about what to tell your doctor....probably the truth is best. Otherwise you'll have to figure out some pretty good reasoning why you want a specific type of bloodwork done more than once (and a pretty good excuse to go with it). I'm sure EVERYTHING you do that you have to show your OHIP card will go on some sort of permanant medical record.
I don't believe a doctor can do anything if you are using a substance without prescription (?? unsure here...). Obviously some doctors are more "open minded" towards these types of things than others, you may get a lecture ;-P