You can't purchase it on a ticket purchased through united.com but you might be able to purchase it by ringing up United and asking them. United.com will only show you the flights as United codeshare segments using United fares but they have the ability to sell tickets with segments on any IATA airline using any published fares, not only flights that have United codeshares. An agent on the phone might be willing to price your ticket using an AC fare in a PE fare class.
You should still get your EQD as the ticket will still be on United stock. In theory United shouldn't care as I believe the route is included in the TATL joint venture so they should get the same money in the end.
Whether they're actually willing to do that will depend on their policies.
Incidentally you might be surprised at the price of AC PE. As the other poster mentioned it's treated as a whole other class and is closer to the price of discount business than economy. I've never flown it so I can't answer whether it's worth it and in any case that's a personal decision.