Hi Roland, what you are wanting to do is possible.
There are a few things to think of though....
Does your machine keep the pressure up full when you are not pulling the trigger?
There are different types of bypass valves so some will reduce the pressure when you are not using them and others will keep it up full - also some machines will have an auto throttle (ours do) so when you release the trigger both the revs and pressure drop down.
You will need to make sure the valve you buy has enough safety pressure to cope with your needs.
We have fitted a valve to the end of the hose which allows us to swop tools over without switching off the engine. We also have a spigot release fitted to the trigger so we can change over the lance parts only. We have a spigot fitted to the large Mosmatic so we can swivel the handle when we store it but it does mean we can do what you want by either uncoupling the handle or tool end at the spigot side. I have 3 photos to show the parts and if you need anything else let me know. Parts are available from most pressure washing suppliers....
Cheers Keith
ECO Pressure Clean