I wouldn't try. From what I understand, if something isn't right the loss is on the FA. (I once asked a NW FA if I could use a CO drink coupon - sounded reasonable to me since it was a CO codeshare flight, and I was connecting from a CO flight (where I got the coupon). She told me she didn't think so, and that if she took it and she was wrong, the loss was on her.) My worry is that if the FA doesn't see the cert is expired, and NWA does, they might try to extract $5 from the FA, which would suck.