I appear to have won.
The Comcast Scientific Atlanta cable and DVR boxes are, according to Comcast, all enabled to use external eSata storage. They only certify compatibility with one WD 1TB drive (which is available quite cheap last I looked). However, they’re careful to state it as roughly “that’s the one we’ve tested”.
Being the intrepid adventurer I am, and having a spare 400GB drive lying around doing nothing, I looked for external drive cases that supported eSata, and found NexStar CX from Vantecusa.com fitting my needs. Bought one for $27 at Micro Center today (about $20 from Amazon last I looked), put the drive in (easy; 6 screws, 4 to attach the drive to the sled and two to attach the sled to the rest of the case), hooked it up–and the DVR immediately popped up a dialog box asking if I wanted to format and use it for external storage. And it’s now blinking away, and our DVR is 13% full.
The drive already spent about six years in my fileserver, so I don’t know how many revolutions are actually left in it. But the case says it’s good for up to a 3TB drive, so I can probably replace this with the next drive discard from my backup set, or something. (There isn’t a way to stop recording on the external drive without losing what’s already on it, though, so the upgrade will involve either catching up, or else losing recorded shows.)