The only difference before and after the multiswitch is that the stretch between the switch and the receiver will have 22 kHz on it for LB\NBs B and C. This should definitely work between the dish and the multiswitch because this signal is identical to the pre-HD signal conditions.

I would only expect possible problems between the receiver and the multiswitch, or that some diplexers might work and some might not. Theoretically, it does not look like a problem, as you see below, but this is Planet Earth where Murphy rules.

Diplexers must pass DC and 950 to at least 1450 mHz through the SAT leg, and about 50 - 850 mHz through the VHF/UHF leg.

Let's look at the DC portion of the SAT leg. The DC leg has to NOT pass VHF frequencies. A tiny cheap little inductor would easily let DC to, say, 20 mHz go through. If they wanted to let only DC to 10 mHz through, the inductor would have to be twice as big, and costs more. DC to 5 mHz, the inductor is another twice as big, and costs even more. So...it is cheaper for the manufacturer to put in an inductor only big enough to make sure VHF does not pass, and they are not likely to spend money to keep, say, 22kHz from passing through.