Unfortunately, UA does not even approach AA's site functionality.

You will have to make a dummy booking by pulling up the flights you want to view seat availability on and go all the way to the information/payment page as if you wanted to buy the itinerary and then choose "Select Seats" to pull up the seat map.

A big pain in the behind. Make sure you to log in if you are an UA elite or you will not see the E+ seats available.