In my scenario, the trainmen would have no involvement with the machines.
The machines would be just like the ones on the platform, only smaller. The only option would be a single ride ticket--no monthlies. Monthlies, weeklies, 10-trips, etc. could still be purchased through your local friendly ticket agent or using a land-based TVM.
Credit cards, debit cards and cash would be accepted. If all options on the TVM are OOS, along with any other TVMs on the train, then the ride is on NJT.
A convenience fee of $2.50 (half the price of the surcharge) would be added to the total ticket cost for using an onboard TVM when the departing station has a land-based TVM or during ticket agent hours (programmed into the software). This will avoid crowding at the onboard TVM during peak periods or at large stations...and the inevitable "I tried to buy my ticket but the line was too long and now I need to get off the train."
Of course, this would have to be integrated somehow into future trains...and probably isn't an option for existing trains.
The technology exists...it just has to be implemented.