Aloha Sp53
I replaced the rear seal in a TR3 gear box, however it was not in the car at the time. The drive shaft might need to be removed to give you room to work in. In my experience, the hardest part of the job is removing the nut that holds the flange in place. An impact wrench is the tool needed to make this an easy step. A car lift would also make it easier for you to get access from below and removing the gearbox cover allows access from above. I would think it is possible and would try it. These are all things you would be doing if you pull the gearbox out of the car to replace the seal.
Safety Fast,
Dave