Beware of the complex solution to a simple problem.
It's a fairly simple matter to run an audio feed from the FOH board to the overflow room, and a fairly simple matter to run a composite (yellow connector) signal from a video camera to a projector using 75-ohm coax cable (RG-59 or RG-6) and a couple of RCA to F adapters. No RF modulator needed.
I'm not sure what the maximum length you can drive with this is, but I've pushed through 100' or RG-6 just from the output of the video camera's composite out with no significant loss of quality. It was an NTSC signal, not HD.
The OP's concern (way back in August, and he ended up just running cables anyway) was penetrating a cinder block firewall. If this was a permanent setup, there are proper materials and methods for penetrating a firewall without compromising safety. Since this was a temporary setup, running cables out the back door is (and was) the easiest solution.