Hello Remco!
In OTK, when registering a client, you can specify the 'Environment' which could say 'sandbox' or 'production'. On a gateway, you would look up that detail either before issuing an access_token (and remember the value as part of the session) or on protected APIs. Based on the value you can either accept or deny or re-route a request, whatever fits your use case.
In addition, you could register a JSON message in the 'client custom field'. Based on that you would make decisions at runtime.
In no case is there a need for changing the client.
I hope this helps,
Sascha