The music acts (last time fallon was at the resort) were open to all int the park at the music plaza stage. Jimmy Buffet, JLo, and Pitbull were there last time. They did 2-3 songs and Jimmy came out and it was over, the majority of the show was taped in the #19 soundstage. That won't be considered a concert since it's so short and not at all geared toward guests (more of bonus). I was there for the Jimmy Buffet one, and it went pretty quick, although he did 3 songs. The sound check earlier was fun too.
Just remember the stage area is set up for TV audience then, so some of the spots were taken up by cameras and boom rigs, so it's not as open up as the regular concerts.