Well - and this is just my style and situation - I'd just have a short conversation with "T" pretty much the way you did here. You'll be potentially in the area and how would she (T) feel if you tried to make contact with M and time & schedules permitting maybe spend a little time together. Emphasize to M that you really want and value her honesty in response because her thoughts are important to you.
Just be prepared for the possible response of "I'd be really uncomfortable with that", because that leads to much longer, deeper conversations. You may want to be prepared to settle with a promise of "nothing intimate on this trip - just a social get-together". But if you make that commitment - STICK TO IT. It will matter in the future.
Just my thoughts....