Hi,
I don’t know if it can help you but i have found something in cpp UE editor code.
You can choose wich plugin will be launch with unreal using -PLUGIN="" engine parameter.
And you can use many -PLUGIN parameter in the command.
I don’t know if you are using rez, but with this, you can create a custom environment variable and generate -PLUGIN for each path in this custom env.
I created an example to work with rez, i share it if it can help you : here