Well, I finally realised that there’s a 0.24 fixes tree for nuvexport that isn’t part of the main MythTV 0.24 fixes tree, so I downloaded that having read on the mailing list that it should work with the MythTV 0.24 fixes FFmpeg.
But no. It doesn’t. 0.24 fixes nuvexport attempts to use options for padding and cropping that are no longer supported by 0.24 fixes FFmpeg.
I was on the point of abandoning FFmpeg and going back to using the transcode option until I discovered that it is no longer supported, so I just had to give things one more try. I installed FFmpeg 0.8.4, ignoring the MythTV version, symlinked it to “mythffmpeg” and installed the latest version of nuvexport from github, ignoring the 0.24 fixes release.
So far that appears to be working.