I would assume that DirectTV would use HDCP copy protection to stop their stream from being copy and any legit capture card usually can't pass HDCP because they are recording devices, which is what HDCP is meant to prevent. That or it would downstream the quality to like 480p or something.
You also realize that you need a PC running full time, so you're spending money on power for that, as well as the receiver running all the time too, and with a capture card you can only record what you're watching, if it works for reasons stated above. You then need software, the EPG as you said from somewhere. You can also only record one thing at a time.
My old PVR for my cable, could record like 4xHD streams at once and do it all while turned off in low power mode.
The concept of doing it is cool, but in reality, a lot of software to setup, finding a EPG stream that doesn't go down, extra power, and again, if the HDCP doesn't get in the way.