Possible, but you could easily wreck your DVR using it while driving. My HOPPER and the VIP922 DVR it replaced both have stern warnings right on the receiver to NOT MOVE while plugged in, and the manual goes into more detail.
This is because the DVR uses a mechanical hard disk drive to store the recordings... but it's not like a laptop drive that is designed to take some rough and tumble. It is a video drive, designed to run for 1000s of hours without complaint.
Pandora is cool, but Google Music all-access is cooler. I have both, and use both regularly, but I'll be using Google on my upcoming off-grid road trip because it gives you unlimited downloads. I can pick all the artists, albums, or songs I like and save them to my phone (32gb phone can hold 100s of hours.) Once I drop off the grid, the music I downloaded keeps playing now from the phone's memory instead of streaming from the cell towers.
Wow. Great info.