I'm not familiar with the system you speak of, however if the switching is all low voltage your options are very limited if you do not want to run new wiring. Ceiling fan remotes usually include a cradle for the remote that can be wall mounted next to or in some cases over the switch. In this case you could hard wire the fan to 120V and just use the battery powered remote for the fan/light.
Personally I would not consider this a workable solution do to the lack of true wall controls, but worth mentioning just the same. I would recommend you at least rewire the switch, feed it with 120V and connect it back to the home run from the fan to the old relay box. Then I would use the wall switch/ ceiling remote unit that allows you to control the fan/light separately.
Alternatively you can rewire the first floor fan/light ceiling box. Yes it involves more work but you should not have to remove any more sheet rock than one 4 inch round hole near the top of each wall per ceiling light - assuming you have basement access to feed the switch. To do this, figure out which way your ceiling joists run, use a hole saw to cut a 4 inch hole near the top of the wall that lines up with the light fixture and the ceiling joist spacing, inside the 4 inch hole drill a 1 inch hole through the top plate. You can now fish the wire up the wall from the basement (or switch if you get lucky with your alignment) and across to your light fan/light. You can even put back the 4 inch plug you cut out when your through using a few screws and a small piece of wood behind it. More work, but if you are redoing everything anyway its easy enough to patch up a few 4 inch holes and you will have all new wiring and no remote BS to deal with. Its worth it IMO.