Select all the games in the platform using Control + A like in Windows, right click and edit, this will bring up the bulk edit tool, choose the emulator field and then the emulator you want.
Since you have them separated into their own folders already this is easy, import the games from their regional folders but import them to a new custom platform such as NES - PAL or NES JPN for example but have them scrape as Nintendo Entertainment System so it gets all the metadata and art. Just be sure and add the new custom platform to the Associated Platforms and command line parameters.
All the command line parameters are the same except you change out the core name .dll file so for Beetle HW it would be
-L "cores\mednafen_psx_hw_libretro.dll"