@paranormalCrystal
I just edited MapInfo.byaml, which contains information on all maps in the game (including some details on test maps not present in the retail copy!), and then replaced the original with cafiine. There's two particular tags/nodes called "Brightness" and "EnvHour". By default, Vss (multiplayer) maps have both set to "VS". Presumably, that signals the game to dynamically load the correct version of the map. Replacing "VS" with "Dark" (Brightness) and "Night" (EnvHour) forces the game to load in the Splatfest mode of the map. I tested both Turf Wars and
Ranked online, it worked completely fine and I wasn't banned. I'm not sure if Ranked will behave properly on other mode and map combinations, however. There was one oddity I noticed while playing: I got the regular randomized battle BGM, but at the beginning the Splatfest match intro music played. If I was playing a Turf War match, the Splatfest last minute music would be played in the last minute.
Patching the file could be completely automated with a script (in fact, I have it partially automated already).
(Also, now that I think about it, we should experiment with some crazy combinations like Dark + Day and Bright + Night)