I personally wouldn't use it or prescribe it during TRT. The point of TRT is to restore low T values. Among the benefits are increased metabolic rate, increased sex drive, slight decrease in body fat, among others. The purpose of TRT is not to get you ripped. Its a therapy to improve overall health. When I hear people considering adding compounds to TRT, it is tantamount to abusing these drugs.
I have no issues running mast or any other compound on cycle, but keep your TRT simple and let it do what it's intended to do. The other consideration for mast is that as a DHT derivative, there is an increased risk for prostate enlargement so I would certainly not condone longterm use. It's simply unnecessary IMO. If you want a lean physique while on TRT, get the diet in order!
Side bar......the is some "speculation" that there may be superior compounds for TRT other than cypionate or enthanate which may have better safety profiles and lower risks at TRT doses or even lower. I will not elaborate at this time because I don't want people getting crazy ideas in their heads,
however, the TRT of today may not be the TRT of tomorrow as research in the area continues.