I know The Cape is already hosting a Halloween party on the 27th.
When The Cape does 2xXP weekend, we generally have less people at a stationary show, and some of the DJs prefer to be out running missions or task forces. Some people like the break though. Still, it's our experience that less people want to stay in one spot dancing during 2xXP.
If you still want to do it on the 20th, be sure to tie it in with the other events, and help each other with the publicity. Get the charity folks and the radio stations all on board together to run a 24 hour event, where people can come and go. Perhaps have a "dance off" at the top of every hour, where people meet up in AP for a 10-minute break, then can team up and go get mad expees again. Actually, that's not a bad idea... would promote community and help people find teams!