For silver members you are met only to be upgraded from 330 I think to 7 days, but I was able to waitlist an upgrade 6 days before the flight. I was waitlisting on 2 separate segments.
The upgrades came through so waitlisting can be useful.

I would suggest if you get an answer you don't like ring again another time as the first person I spoke to wouldn't let me waitlist on a flight 6days out but another person let me waitlist.