I've tested this over the past few weeks, and I think I found a "glitch" with the CO ticketing system.

I didn't see this in the other UDU thread (mods please feel free to merge).

1) Book a CO codeshare on UA metal. Make sure you provide your UA MP number. Ignore the fact that continental.com doesn't let you select seats. Note that there's a bit of a time delay between the time you book it on the website and you get the email confirmation. I think this plays a big part.

2) Log into united.com, go to My Itineraries, and then select your seats. As a UA agent explained to me, once you have a seat selection (from UA) you automatically get added to the UDU list.

If you don't select your seats in the UA system and you simply let the CO system pick the seats based on your window/aisle preference on continental.com, you won't get automatically added to the UDU list.

Again you have to select your seats on united.com immediately after booking. If you get the eTicket receipt from continental.com in your email, its too late and the CO system has already assigned you seats.

I've tried this several times over the past couple of weeks. It works. If I don't assign seats myself via united.com, and let CO assign my seats, I don't get automatically added to the UDU list.

Does anyone else have similar experiences?

I suspect soon UA will get wise to this and start conducting UDU "sweeps" similar to the E+ "sweeps" that are currently done.