You write "the" latching switch as though we have discussed it here. I missed it. What are you referring to?

Have you been able to find a document for the processor similar to the doc you found for the projector?

Does the projector have an IR sensor that turns itself and the processor on? Does the processor have an IR sensor that turns itself and the projector on? Does the HDMI connection provide power ON and OFF to the other unit?

Backing up one step (which is to say not attempting to answer your question, but instead looking for a different solution):
I take it you're using Elan products, which I'm not familiar with. Is there a way to get IR to the processor or projector? If there is, then a macro on a control interface would turn on both the Arcam and the Epson.