Yep, I've done this several times (three times, I think). Twice were last October during the huge SIAL exhibition in Paris. Had to stay at the Hyatt CDG. Booked the night before the exhibition at EL stay certificate+the next night at EUR 300 (so at least I get one stay credit out of this). Then, checked out and went to the PH Vendome for 2 nights. Then, came back to Hyatt CDG for the last day of the exhibition at EUR 270 (I think), and added on one more day after that with PR stay certificate.

This year in January, was also able to use a CH certificate+one paid night+PR certificate for total three nights at Hyatt Regency Cologne during the ISM exhibition.

So, it really does work if you spend the time to search the different days and availabilities on the website.