I have thought about this as well . . to pay for the middle seat (of 3) for me and my niece. Makes a big difference to have that extra space. I think it's possible but not sure about the PQMs . . but then I don't really care that much. I think you have to check in with BOTH boarding passes so they don't use that empty seat. I sat in the aisle of a 3 seat on Southwest and the large man at the window had his "EXTRA SEAT" boarding pass sitting on the middle seat so I knew. Worked out great for me!

Yes UA allows the purchasing of an "EXTRASEAT" -- can not be done online, you will need to call. It is a reserved seat and can not be ignored by UA without following the procedures for any other ticket.
More details in the wiki,

You will per the thread's wiki

I think it is important to point out to the OP that will need to exercise a certain amount of vigilance in defending their extra seat from poachers and either lazy or aggressive staff - for example, on an overbooked flight, the GA may try and strong-arm that extra seat away, or an FA may tell someone to sit there, ignoring the claim of the OP - sure, they will be compensated after the fact, but that is little comfort when you're about to sit for 14hrs on a long flight. Other passengers may covet the seat (less likely with a couple booking an empty middle), and you may need to get aggressive in keeping them away. Not a sure thing that trouble will happen, but the OP should still be aware and vigilant, at least until the flight is underway.

VERY well stated!! Thank you!
Dave

I have successfully self booked EXTRASEAT online.

After reading the warnings here, I called in, twice, to verify and was met with puzzlement by the agent "Everything EXTRASEAT is already in order" EXTRASEAT was still there on connection and return. Perhaps I just got lucky.

Coud you describe the steps you took?

Yes, please tell . I have mixed award and paid seats, and just get two PNRs. It seems to take the 1K agents a while, but it always gets done.

In the past I've booked LASTNAME/EXTRASEAT online, but the last two times when I've booked it by phone, the agent booked it as LASTNAME/EXST. The first time it was booked this way, I was able to view the reservation online using the locator and LASTNAME. However, for the most recent reservation I made, when I try to retrieve it by locator and LASTNAME, it says this itinerary cannot be viewed online and that either the confirmation number or the last name is invalid. Of course I've double checked this information against the email confirmation, but I cannot get it to display online. Should I be worried? Everything looks OK in the email confirmation, including the seat assignments. I'm somewhat worried that I won't be able to checkin when the time comes. Note that I booked the EXST using miles, so it's in a separate locator from the primary reservation.

Did you get your question resolved? I'm about to do the same thing - 6hr flight, GPU upgrade pending, so I want to book an award seat to block the middle in my exit row, but it's only for one segment of my itinerary so I believe they need to book it as a separate record, but not sure how to handle check-in, boarding passes, etc or how the seat will show as "boarded" once I board with my primary boarding pass.

My most recent EXTRA SEAT booking is in the record under a separate PNR: EXSTLASTNAME You should be able to see it on line, and we print a boarding pass so that there is no confusion.

It took some doing, but reservations was able to make the EXTRASEAT booking for me as a separate mileage reservation for AAA-BBB only, then connect it to my revenue ticket for AAA-BBB-CCC. I can view it online correctly, but I will need to check-in both reservations, and print boarding passes for both, and scan both during boarding. I also need to watch the booking carefully because any schedule or aircraft change will scatter the seats, but I'm still hopeful the GPU upgrade will clear on the first segment, so I can cancel and redeposit the EXTASEAT mileage booking. Two second row exit row seats together will be a decent compromise if the upgrade doesn't clear.

With the armrest fixed in exit rows, this will be "more personal space" but not "more seat space"

Shoot, I totally forgot about the fixed arm rests on the 738 - for some reason I thought they went up. Just to be safe, I'll move "us" further up in the E+ area, thanks for the reminder.

I still can't view the EXST reservation online, but I had an agent at the United Club print out the EXST itinerary, and everything looked OK. Like you, I did it this way because I have RPUs pending in both directions, but if they don't clear, at least I'll have the extra middle seat in E+.

Lifetime miles
 

I see in the Wiki that no PQM for the extra seat. Does anyone know if you get lifetime miles for both seats if you pay for the extra seat?