Due to some poor planning I ended up with an extra M780, instead of throwing it on e-Bay I decided to give the beta QAM->ATSC hack a try.  With the most recent drivers the tunning and picture quality have been very reliable, unfortunately the guide data would go away every day or two just like my ATSC data did.  The cool thing is that the same approach for keeping the sub-channel guide data for ATSC works for QAM as well.

Steps:

1. Do the QAM scanning as documented in the Avermedia docs
2. Get out a piece of paper and write down the name of the channels as they relate to the station identifier (35-1 is WLS-DT for e.g.) and the channel number (i.e. 1301).  You will need to count up the channels to get the correct subchannel (the first instance of ch-35 is 1, the second is 2, etc).
   
3. Close MC, open up the atscprefs.xml (C:\ProgramData\Microsoft\eHome\EPG\prefs) and find the channels by major/minor channel and change the callsign and userAssignedName to the correct station identifier.  I used the station id + "-DT", but you may need to play with it a little (omit the "-" if it doesn't work the first go).
   
4. Restart the MC service processes
5. "Add Listings" to the channels like you normally would