I tried changing the Emulator App Path but it keeps resetting back to "..\Plugins\SteamLauncher\SteamLauncherProxy.exe" when the SteamLauncher setting is turned on.
I'm guessing that this SteamLauncherProxy.exe is supposed to be used in some way but I have no idea how to indicate that rpcs3 is to be used as well.
Similarly, I tried this on PS1 and PS2 games as well with the same result. I feel like I'm missing an obvious step here.