Anybody know if the analog out (s-video) of the PD-170 is pre, post or partial compression?

I don't absolutely know, no.

However, if it follows standard Sony big camera practice, the S-Video is a pure analog signal coming from the camera head (camera in Camera Mode). From tape, it is obviously 'post.'

It probably has been passed through the recording electronics in some manner so it isn't like the big camera output that is direct from the camera head. So it shouldn't have been compressed and then decompressed. On big cameras, it is called E to E output.

Easy way to tell (maybe): Set the camera on a tripod and evaluate a scene with the camera in Camera Mode and then do a recording and look at the playback output. Pick something with some color (like a color wheel) and something with fine detail (like a resolution chart) You should be able to tell a little bit of difference . . . maybe.