I think by now most major manufacturers are offering some models of TVs and AVRs with ARC.
I suspect that ARC works just like any other digital audio output on the TV. It just eliminates one cable.
To find out how HDP functions with ARC, you will have to check the the HDMI 1.4 specification, which will be difficult considering you are not an adopter. It might be easier to find the information in documentation from one of the HDMI chip makers. I think it is irrelevant, though. The AVR will likely play whatever comes over the ARC when the TV is selected as the input, just like it will play whatever comes over the Toslink or coax cable when that is the connection and it is appropriately selected.