There are SN agents at BRU but whether they will assist or not is a crapshoot.

See if you can find Dirk, one of the UA CS leads at BRU. In the same situation as you with misconnects SN-UA at BRU I've several times been rebooked via IAD which leaves much later than EWR. The departure order is EWR, ORD, IAD...so move from one UA gate to the next to elicit necessary assistance.

You could also try UA CS in the USA. Last time I was stuck on the downtown to Zaventem train - this was in Oct, there were 2.5 hours of delay after breakdown between Scharbeek and Zaventem, incredible for SNCB - and I missed both EWR and ORD. I called UA 1K on the train and they fixed it for me before I needed to engage in checkin or gate agent battles, including preserving my PZ seat.