James
The Scorpion has in its clean tank two hoses - a water pump suction and a return pipe (so that when you not wanding the water is not building up pressure but returns to the clean tank).
These 2 hoses are long enough to come out of the tank and you could dangle them in a bucket of water by the side of the machine.
You could have a prespray in that bucket and just clean water in the tank.
You need a lance and then you can prespray from the bucket, then put the hoses back into the clean tank ready for wanding.
Lots of good design features of the Scorpion.
Maybe Matts Airflex is something similar.