It looks like the go-to guys for DTV hardware are the SR Systems folks in Germany. I traded some e-mail with them, and it turns out that their minimod system does null-stuff the transport stream to 19.38 MB/s, so their 8VSB modulation should be tunable by the HD HomeRun. Very exciting!
I traded some e-mail with their software guy, and while he didn't promise anything, it seems like it wouldn't be too much trouble to add a set of static PSIP tables to the transport stream. If they were able to add support for even a low bit rate monaural AC3 audio stream, then that would make their system completely compatible with ATSC! That would be a win!
In other areas, I think I did the math wrong the first time. Their minimod puts out 107 dBµV, which at 50 ohms is about 0 dBm. Feeding 0 dBm into a DEMI 60 watt 900 MHz amp would yield about 15 watts of output. That may not sound like a lot, but the really good news is that that level of output would be squarely inside of the amplifiers linear response range. One of the things that's absolutely required for DTV is that the power system be absolutely linear. When you drive an amplifier to saturation, it stops being perfectly linear, and that would trash 8VSB.
15 watts of output into a 12 dBi antenna winds up being over 100 watts of ERP. If you can count on the receive stations having 15 dBi of gain in their antenna systems, then that's likely to be a 20-30 mile (line of sight) service area. That's not bad!
Of course, it's all just predictions and hot air until you actually get hardware. But at least the predictions and hot air don't cost anything.