Finally got the chance to try this. It actually does work, although it's probably not worth the effort unless there's a must-have subchannel on VHF in your area that DirecTV doesn't have. The DirecTV box works fine as far as I could tell, and I got two VHF channels from OKC and three weak ones from Tulsa.
I used the old Pico-Macom UVSJs. Those block power on the UHF side, so they are not recommended unless you are diplexing a non-power-carrying branch of your DirecTV "tree;" but that was the case in my in-laws' guest bedroom and I wanted the option of adding an antenna preamp to try to improve those Tulsa channels.
No time to do that tonight, though. Heading north to try to catch a glimpse of the total eclipse Monday.
Edit: I did get to watch the eclipse. BTW, if I were going to do this "right," I wouldn't bother with diplexing at all. I'd put the DirecTV box in the attic, along with a Next Generation RF system (so the remote control could still work) and an ATSC RF modulator. Then the whole DirecTV "experience" would merely be one extra channel along with all my OTA channels. Only problem is the modulator would make that solution rather expensive.
This was mostly a theoretical exercise: can I make diplexing work at all, even after DirecTV seemingly went out of their way to design a system that wouldn't allow it? (Dish's "Hopper" should still allow some UHF diplexing, especially after spectrum repacking, but not DirecTV's "Genie" system.) Turns out I can, FWIW.
Last edited by JHBrandt; 08-31-2017 at 08:16 AM.