will you suffer the devices/gears cannot detect the signals from remotes?
putting ALL components to the back is surely no problem but just because to comfort you not to use a long HDMI between projector and av-amp seems not worth it.
why not putting a large cross section area trunking inside the wall, ceiling/floor so that you can easily hide the cable and be able to change it in the future.
I put all my gears at the back, all remotes are working perfectly. I use a 10m sub cable to put the sub at the front. And I believe in general, hdmi is much more expensive than speaker cables. Also, speaker cables are easier to turn inside the wall. So, I don't think there are anything wrong to put amp at the back.