Last year, I was traveling with my parents and sister (two separate PNRs) on a simple DEN-BWI-DEN trip. At T-24 for each flight, I saw decent E+ availability, so I called in and explained I wanted my parents (different PNR) to get my E+ entitlement. Both times, the CSR said, "How about I just move all four of you to E+?" That was nice.

tl;dr - study the seatmap at T-24 then call in. You might get lucky.