As previously mentioned, you have to call in to book AC award inventory. AC does not show up online, only US and CO do, in the "star alliance award" category on the website.

When you say AC has "numerous wide open flights," what do you mean by this? Are you looking at seatcounter, or just noting that there are lots of flights with prices that are fairly low. If they are indeed wide open for revenue bookings, this does not mean there are necessarily lots of seats that AP is offering at the regular price.

If you are looking on the Aeroplan site and the flights have lots of availability, in which reward category? If they are in the lowest price category (can't remember what they call that now), which would be 15,000 for short-haul within NA, and 25,000 for long-haul NA, then UA certainly has access to this inventory (no star blocking on these routes, at least there hasn't been in the past that anyone has reported). If they are on the higher level awards (i.e. at higher than the 15,000/25,000 mile award levels), then these seats are indeed limited to Aeroplan members (they'll let you book the last available seat, but at levels no one should be willing to pay - saw 115,000 for an ORD-YYZ round trip once). No other partners have access to this award inventory.

If you still do want to try and make a transfer, there is a tool on webflyer that will show you the ways you can transfer from one program to the next, if there is one, but these often are not a simple thing (might have to go through 2 or 3 + programs to get there) and are almost always a waste of time, energy and will result in many less miles in the account after the actual transfer is complete. Not sure if there is a way to go from MP to AP personally, but you can check it out there if you want to.