I've never tried an AC flight pass either, however, theoretically, they book into whatever Tango Plus or Lattitude fare class they are set to (depending on the pass you select), and should go into one of those regular revenue classes. I can't see why you wouldn't be able to use them to collect UA miles.

Hope you do realize that AC passes will only allow you to book on AC-operated flights though, not any partner flights including UA's. Perhaps in irrops they can make an exception, but definitely expect AC metal only. If you're going SFO-YYZ, probably not big of a deal as there is only one UA nonstop anyway, but if you are doing, say, SFO-YVR on a regular basis, that may limit your options to about half the available flights.