Jack, I get it now! I would suggest you try what you want to do, but with one exception:

Feed the modulator into the VHF/UHF connector on the diplexer at the theater end, but use another diplexer before the multiswitch at the upper end to "filter" the modulator signal out of the coax before it gets to a multiswitch output connector.

I would not feed a signal into an output, as there is circuitry within the multiswitch, besides the diplexing, that is probably not two-way capable, as it's most likely not passive.

Please let us know if this works. I'm sure there ae others who could benefit from your experience. That's why we're here!

Larry
www.fineelectricco.com