After way (WAY) too much experimenting, I just found a way to get XVideo overlay active on DFP-0 and not CRT-0 (CRT-0 is still the primary display and DFP-0 is the secondary, but that's actually the way I wanted it, since DFP-0 is my LCD HDTV).
After so much time trying to find this answer online myself and having no success, I registered on nvnews just so I could post it.
The trick is to:
1. use separate X screens (not Twinview)
2. when you set up your separate "Device" sections, as per the nvidia README, there will be a "Screen" variable in each set to 0 or 1.
The key thing is to have the "Screen 0" be in the Device section for the screen you want to get the XVideo Overlay, even if that Device actually corresponds with what X considers to be screen 1 (i.e. display :0.1).
I get a little weirdness with the overlay showing video only on the top and bottom edges sometimes in MythTV, but if I just stop and then restart playing it clears it up.
I hope other people find this useful.... just take the config options from NVIDIA's readme Appendix P
and then adjust the "Screen" options as necessary so that Screen 0 is the one where you play most video.