Jack, the multiplexer is "powered", even if it has no external power supply; the receiver's send a DC voltage up the line, remember? It is used by the multiswitch. I have a 3-in/8-out, and it's sending out seven signals, without using a separate supply.

I don't think anything will be damaged by trying your idea. The diplexers internally block DC from the antenna connector.

Larry
www.fineelectricco.com