It looks like my -makemovie change was rejected by MAMEdev. A patch I submitted after the -makemovie change was accepted, so I'm pretty sure about the rejection.
It's possible, though, that they'll add it after I better explain the motivation behind it.
I think I'll rename the option -mngwrite <string> instead of -makemovie. That way it is similar to -wavwrite which is already in MAME.