I'm sure the meet and greet will probably be available during the party although nothing official has been announced yet, but I wouldn't count on the line being short. The more popular party characters still have long waits- sometimes a couple of hours even. We have gotten stuck waiting over an hour to meet the Princesses and their Princes during the party before (the line was posted at 30 minutes which was why we got in). Back when Belle met with the Beast during the party, their line was 2 hours one night when we checked and that was within minutes of the party starting. We had put another high priority character first and then tried to run over to see them. If they are your top choice as opposed to one of the other high priority party characters, you might try getting in line for them 1 1/2 or 2 hours before the party starts- then hopefully you will be near the front and not missing much party time in line for them. We did that for Jack and Sally at
MNSSHP party last year- they had 2 hour lines during the party most nights.