Another data point for the discussion: booked a UA flight LGA-ORD-LGA yesterday, during the res process E+ was shown as $39 / segment, no E- seats available on one of the segments (often the case on this route, I've seen).

Website told me a seat would be assigned at the airport (also normal). However I checked back 2min later at the reservation and saw that I'd been assigned an E+ seat.

Called the UA res line and was able to get a different E+ seat on this leg (wanted window vs. aisle) and E+ on the return, took 2min and didn't have to mention my CO status - the agent mentioned that he could see it.

Now I'll have to see what happens with upgrades