I can't think of an instance where irregular operations on WN was anything other than first come-first served when being rebooked on the next available flight. Keep in mind that those pax booked on the flight you will now be on have (presumably) checked in at their 24 hour mark and will already have boarding pass numbers before you get your BP. This is not all that different from the legacy carriers: if you were in seat 12A on a cancelled flight and another pax has already been assigned 12A on the flight you are rebooked to, you can't also get 12A. The one potential benefit for elites on legacy carriers: they may have already proactively rebooked you on the next available flight, using the seat preferences in your personal profile.

While it doesn't appear to have been the case in the situation you experienced, as everything they (currently) fly is a 737, WN will often do plane swaps to effectively re-instate the cancelled flight and it will go out with the same flight number as originally scheduled. In my experience, while it does happen, the legacy carriers are less likely to swap in a substitute aircraft.

Personally, if my confirmed flight is cancelled, I consider myself lucky if I can get on the next flight and I don't worry much about my boarding pass number/seat selection.