My first question would be why??? "Is there provision for them to move between the 2 channels with the same content" by this do you mean if you drive out of range of one transmitting tower and into range with the next. I would think that there would be a delay in the decoder getting the transport stream settings and the two transmitting towers would be on 2 different channels. DVB-S is really the only mobile source. You would still have to be stationary. ie Camper Vans, Mobile homes etc.
For a car tv. No point getting one thats an analog tuner at this stage, so I will probabbly skip the tuner and see whats available in a years time or so.
There are plenty of mobile mpeg2 based tuners online to buy, but nothing mpeg4 yet, and the settop boxes only support one antenna vs the 4 that a normal car tv tuner has on it so I dont know how well it would go.