From what I've read (e.g. think like a pancreas) 50% of the carbs in sugar alcohols and similar sweeteners still make it into the blood stream so I would recommend to not have them if you want the OGTT to be accurate.
Why don't you talk to your doctor about this? Either to find a way for you to do the test (maybe using ice chips might work?) or to find an alternative to the OGTT?
I'm sorry, but visiting an online forum where everyone can write anything looking for people to tell you that it's ok to go behind the docs back (I'm guessing that you hadn't planned on telling them about the mints - sugar free or otherwise you were going to have during the OGTT) and invalidate the tests is not a smart thing to do.