Man, getting a straight answer on this is surprisingly difficult. I did some google-fu, and here's what I have found.
Comcast uses DVRs from Motorola, Scientific Atlanta, and others, which absolutely have functionality to use esata storage. You can actually download the user manuals from Comcast's site and see the eSATA port pictured, with the caption "for external hard drive storage."
The functionality is there. Now, Comcast can be anal retentive, so they may have deliberately disabled this feature. There's no mention of it on their website that I can find. Furthermore, I found this on a Comcast Q and A blog.
http://xrl.us/bifyqv (Link to blog.comcast.com)
Basically, the Comcast guy (2nd post) is saying that the functionality exists on at least one class of DVRs, but is unsupported-- yet it should work. In fact, Comcast actually checked to make sure that it did.
That doesn't sound like a blanket refusal to allow the functionality, it sounds like they don't want to do tech support on ten million yahoos hooking up random equipment to their machines.
I still think it's worth checking into, either by going on line to someplace like avsforums, or calling Comcast and simply asking. But it may be a bit more complex than plug-n-play.