While this shouldn't be considered a standard, but I know of a number of short lines using Mototrbo mobiles just mounted on top of the dash board. I imagine with a remote kit you can put the head and the radio package anywhere you want. As far as repair access I guess it would depend on where you mount it.
Our M-7 electric cars which use GE/Harris Orion/M7100's mobile radios have the radio package(radio and power supply) mounted in a ceiling cabinet above the engineers position. The old deisel FL-9 locomotives had AAR heads in the dash and Micor mobile radios mounted in the engine bay below the engineer deck, which was a PIA to get at. I think a lot of where the equipment goes depends on the vendor building the car and sometimes has to be based upon where there is room. The M-7 equipment ended up in the ceiling because of future proofing space for PTC. Which is funny since now the PTC stuff is in the ceiling too, and not in an area that was set aside nearly 20 years ago.
Clean Cab standards were nice, but considering the costs of new equipment standard mobile radios are about half the cost. As long as you have a 12 volt supply.