Using the latest ZVBI CVS version I can get full support for captioning (NTSC) on the PVR 150 card (on the command line with zvbi-ntsc-cc). However, no matter what I try, I can't get anything to show up in mythtv.

Is there a way to make mythtv use the ntsc-cc app to get caption data instead of what it it currently doing?
It's really annoying to know that captioning is supported 100%, but the program mythtv uses (/usr/local/bin/vbi) always returns the same error "vbi: Invalid argument."

Any programs out there to "fake" captioning, so that it overlays over mythtv video?

Well if the command line interfaces are the same you could always move the /usr/local/bin/vbi program aside and them create a symlink from that to the one you want to use... I'd double check that it's not a setting you can fix first though...

Unfortunately, they are not the same. zvbi-ntsc-cc (completely compatible with ntsc-cc) is part of the zapping library, while the vbi app comes with the ivtv driver (and seems to have little or no documentation)

Well that leaves throwing a shell wrapper around it that records the comand line parameters or taking a look at the source code...