I tried using UA MP miles to book C class upgrade IAD-IST this past week. Didn't end up doing it. Spoke with 3 TK reps. Two said C seats were available, one said they weren't, so I never was sure about availability. Second, you can't book it with TK. Need to buy a TK Y/B seat first, then go to UA to request the UG. Not worth the risk.
Did look at using just miles to buy the TK C seat on UA and CO websites. Never saw the TK flight available in C. Available in economy, though.