Jameson -
I have the standard scientific-atlanta HD-box from Time Warner and as of 0.21 mythtv, the only HD capture options out of the firewire were the local ClearQAM channels (same ones you can get on HD-Homerun). The upper content is all encrypted and therefore not streaming unencrypted from firewire port.
However, you should be able to (in theory) use channel changing via firewire, and put the output from the digital tuner into one of your Hauppauge cards to capture the other premium or specialty channels outside of the normal spectrum.
I use the basic service, 2 Hauppauge pvr-250s and the 2-HDHOMERUN tuners on clearqam over the signal. I'm not sure why you chose to go over the air instead of just getting the signal via the analog cable line on the HDHR. 0.22 mythtv introduced multicast ability for the HDHR so it functionally can record quite a number of streams simultaneously because CBS & NBC share a stream on time-warner I'm pretty sure. I've had as many as 5 captures happening on the HDHR at once.
Your other option is to look at one of the newer hauppuage devices for HD capture directly from the cable box. I'm personally waiting until the silicondust people get the cablecard device out in the next year or so. Hopefully myth support won't be too far behind. Then in theory all Digital channels should be available for 4 or 5 a month for cable card from TW.
Ryan
P.S. Mythdora 12.23 is due out in a week or two. Provided march 1st is still release date for 0.23..