I'm not sure I follow how you're doing your current workaround, but how about trying something like this:
Add new Device, Select Radio Tuner (or should it be TV?), for Manufacturer, pick -not listed-, then enter "custom" or something, then enter your model "DRX-702" or whatever. The review page will probably list the manufacturer as "AAA Generic". Teach your Harmony what it needs and see if you can still set Favorite channels.

A generic device will stay generic so you won't have to worry about them adding it eventually, as far as I understand.