Ha ha ha.

(1) Booked a 4-passenger ticket for me (Plat), my wife (Plat), and my two kids (no status), CLE-IAD-BOS.
(2) Checked in online via the web app. This allows us to put all 4 pax on the upgrade waitlist! We say that all pax want to be upgraded and that my companion is my kid, my wife's companion is the other kid.
(3) Now we show up online as #1 (me), #2 (first kid), #3 (wife), #4 (second kid) on the upgrade waitlist.

Overnight:

(4) I clear into PZ on CLE-IAD. The PNR splits -- now the original PNR has my wife and two kids, and the new PNR has me.
(5) The upgrade list for CLE-IAD now has
#1 first kid
#2 second kid
(6) The upgrade list for IAD-BOS now has
#1 second kid
#2 me
#3 first kid

My wife has vanished from the upgrade list altogether. ?!.

So I think the lesson here is that with a complicated CPU-eligible multi-pax PNR you should not check in in advance, because the auto-splitting for advance clearance will randomly throw people off the waitlist.