I think I semi got it working following some instructions from the FLIRC forums here:
The only issue that I’m seeing now is before I attempted to try to get everything configured “right” using this page as a guide, I actually just plugged in my FLIRC without doing any setup or configuration - it was configured prior to control a laptop Kodi install and was set up on my Harmony with the standard FLIRC device. The DVR control buttons actually worked in Channels DVR with the stock commands there, but obviously I had no back or home button for the Shield mapped out yet.
I followed ixian’s instructions in the thread and added a new device to the Harmony, per his recommendation an ATV-163 and Nvidia Shield. Added a new activity called Shield and added both of those devices to it, set up my inputs, etc. I then went into the Flirc software and selected the Nvidia Shield profile and mapped out the usual directional stuff, select, back, and home to buttons on the Harmony while sending IR commands from the newly created Shield activity. Then flipped over to the Kodi profile in the Flirc software and mapped all the DVR control buttons , play, pause, ff/rw, skips, etc.
These buttons worked as advertised inside Kodi but didn’t work inside of Channels. Must be something about the difference between the codes sent in my Harmony / laptop / FLIRC device profile vs. the Harmony / shield / View ATV-163 device… I’ll figure it out. Just knowing that there’s SOME commands that work to control the ff/rw functionality during live TV without bringing up the UI all the time is enough to go on.