i used a handpumped backpack for a comercail job BEFORE i built all the other kit, it took 2 of us though, 1 with the backpack on and pumping, me with the 9 M unger pole fully extended

that was the way to jump into WFP action

did the job for 2 cleans though, by that time had enough money to build the WFP setup i have now

I stil use the handpumped for a few accounts where they have loads of steps to the rear of the house, only a few windows on each though AND 1 of the connection on the backpack leaks a little, so now i put it on a chair or table and pump till its full of preasure then clean away, saves me getting a wet back

( i really should try and find the leak)
it can be done for the odd few windows