Line in on a camera only works when you are recording from an external video source. If you are recording through the lens, then you only have a mic input, if one exists. A lot of the mini-DV cameras have no mic input. That's one of the things that I was careful to look for when I was looking for cameras. Only a handful of the Digital-8 cameras have mic inputs too.
If you are only recording audio on a video tape, then I think you can run line in and just play like you have an external video source. I don't know if the camera would get upset about never getting video synch, though. I wonder if it would shut down after awhile if it was not getting video synch?