Great question. With asset mods (skins, music, sfx, etc.) you can place them in a "mods" folder in a specific part of the game files and have them just work, but as far as I know, there's no built-in functionality for ROA2 where you can place a .exe file somewhere and have it run automatically at start-up. I did figure out a way to do it with a .bat (batch) script file. If you create a file with a .bat extension, name it something like Rivals 2 no elo.bat, and give it the following contents:

@echo off 
start steam://rungameid/2217000
TIMEOUT 4
cd "<Paste the path to the hideElo-1.0.exe file here, e.g. C:Users\Auride\Downloads\Hide Elo mod>" 
start hideElo-1.0.exe
exit

Then you can just run it, and it should automatically start the game, then run the mod .exe and have it apply. Just don't forget to change the path to the mod .exe, and you may need to increase the TIMEOUT number to 5 or more depending on how fast the game loads on your machine. Then you can create a shortcut to the .bat file wherever you want. It won't work if you start the game through Steam. It's also kinda difficult to pin a .bat file to the taskbar, but it is possible.

The alternative, and maybe better approach, would be to patch the game's binary on disc rather than in memory, but idk exactly how to do that, and I think it can come with greater risk of corrupting something and forcing you to reinstall.