Thanks @angel and @alex. We do that, and it does work great for dedicated desks and 1-desk offices. But we've got 60 offices that are 2,4,6 and 8-9 desks. Typically, our Commons members aren't looking for large spaces, and the professionals who are need at least some private, dedicated space - joining as Commons members doesn't work for them.
We've been adding these folks to waitlists by office size (and keeping in touch, inviting them to events, etc). Several lists were 20+ deep. This seemed to be a great indicator of interest and gave us confidence that we'd be able to refill offices if needed. Pretty quickly, that proved misleading: when offices open, we find ourselves scrambling through long lists; giving each person 48 hours to confirm can easily take a full month to find our new member - a very nerve-wracking thing when it's a bigger office.
We're considering a fee for non-members to be on a waitlist. If they take the office,it would go towards their membership.
Not sure how much that'll help...interested parties may have found other space in the interim... but we thought we'd try and wondered if anyone had any similar experience.