Communicating via USB Port
There is a command protocol that uses the front USB Port. I played with it successfully on the HR2x boxes, but haven't tried it with HR34/44.
You need to use specific brand(s) of COM/USB adaptors since it needs to match the built-in drivers in the DTV box. The COM runs at a pedestrian 9600 bps (please don't refer to it as 'baud'). Since I no longer had computers with 15-pin com ports, I used the same COM/USB connector on each end with appropriate gender swaps.
The description of the commands, etc., can be found in a DTV document: DTV-MD-0058, "DIRECTV Set-Top Information for the Installer." A web search will turn up several sources. The most recent version I've seen is 2.1, dated August 6, 2007
The command set includes status inquiry, functional requests and simulation of a remote, button-by-button.
Have Fun!