You can diplex the OTA signal onto the satellite RG-6 by using a 5x8 multiswitch, four satellite cables from the dish plus one OTA connected into the five ports of the multiswitch, that will allow you to use four cables into the TiVo room all carrying satellite and OTA. Then just use a diplexer on one of the cables to break out the OTA signal from the satellite signal.
This will work until you have to upgrade to the new dish/receivers for MPEG-4. Then the diplexing won't work because the OTA signals are in the same band as some of the new satellite signals so you will be back to solving the four wire problem. But at least diplexing will give you a year or more before you have to run an additional cable.