It's possible for compressed air to generate a static charge and damage components, but I've never actually seen it happen. It seems more likely that something else happened when you disconnected/reconnected the cable. Perhaps there's a cold solder joint/mechanical issue where the socket joins the circuit board.
There has been other unusual behavior on that port, like not lighting the LED on the Sling adapter, but I had attributed it to using an unpowered USB hub. Now it doesn't work with the EHD even if it's direct connected.
I've heard that about compressed air but also never personally experienced it. Of course ESD is a very arcane science. Will try to remember to check check the port for mechanical damage in 6 months when I do the next dust blow-out. For now everything is working OK from the front port, just don't like the aesthetics.
Did a careful blow-out with a new can of compressed air and am happy to say the rear USB port on my 722 is back to full operation. My conclusion is enough residual dust must have entered it during the first dusting to disable EHD detection. Thanks everyone for your help!