I have used a backpack, the 20L hozelock one. It has the hand pump on the side, and you can pump water successfully up to at least 40 feet, the only problem I found was the amount of water you got out the brush, even when working on lower windows. Although it done the job, you were hanging around a bit too long waiting to pump enough water to rinse the window properly.
It can be a bit awkward pumping and using the pole at the same time, and it can be a bit of a nuisance pouring water into it all the time, it is heavy on your back, but it does the job.
Anyone interested in trying it, you just cut the lance off and join the tubing supplied, to the tubing from your water fed pole, fill with pure water and away you go.
Peter Fogwill