ShrineFox from the Amicitia forum recently dumped the game data for PlayStation Vita games Persona 4 Golden and Persona 4: Dancing All Night, granting access to hidden data in those games.
Within the game files for Persona 4: Dancing All Night, an unused Persona 5 Velvet Room alternate costume was discovered by @regularpanties.
This unused alternate outfit in Persona 4: Dancing All Night is the same as the outfit that Caroline—one of the Velvet Room twins—is seen wearing in Persona 5, in the PV02 trailer.
The file name for the Velvet Room costume in the Persona 4: Dancing All Night data is “i01_05E.tga,” with no distinct playable dancer associated with the outfit. However, it is likely that the costume was designed as an alternate outfit for Nanako Dojima.
Comparison animation of Nanako P4D costumes: