Never tried wearing a condom for a HJ. Might be interesting . . .
Keep in mind that the massage oil used by BGs will disolve the latex of the condom, so it might fall apart during some serious wrist action, which would defeat the prophylactic purpose of wearing one.
If you want to wear a condom for a handjob, you would have to bring your own water-based (not oil-based) lubricant for the BG to use. Watsons and Mannings sell these lubes from the same shelf location where they stock their condoms. Note that these water-based lubes can get dry and sticky during prolonged use, so the BG may have to add some water or spit on her hand to make things slippery again.
How you get the tube of lube from the locker room into the massage room will be your challenge.
If you have a regular BG that you like to visit, you can ask her to keep the lube in her locker for you.
For you bros that cap your bro with a dom for other action during an oily massage session, just make sure you don't get any massage oil on your dom before sticking your bro where other bros have gone before.