I've been making a note of the coordinates of "farm" planets (those of inactive or banned players) for my own use. I was wondering if i can pass this information on to members of my alliance via pm, or would this contravene the no posting coordinates rule?

I think it is allowed via pm.
If it isn't how can we coordinate ACS?

The rule in regards to posting coordinates only applied to the Forums. You may transmit coordinates to your alliance mates without worry of punishment- unless of course, you send them some bogus coordinates and they end up losing fleet.