Yeah the RO filter is what really knocks down the pressure from what I am told. My system is mounted on my trailer, which makes it easier to set up and pack up, but requires a significantly longer hose run. With your set up, you should be able to have your DI tank right below where you are working and hopefully the house has enough pressure to push it vertically and still give you a good stream at the end. You might be able to set the DI on that second level porch and get the aid of some gravity on your output to boost pressure a little.
From what you have pictured, that looks like enough water flow from your rinse bar to get the job done. Do you have a multi-stage/little giant ladder by chance? It looks like you have two porches that you could set the ladder up in an A-frame style and have a stable enough surface to stand on and reach the center of the windows. I have a 22ft Gorilla Ladder that, when folded in half, gives me a solid standing point at 11ft. You should have enough leverage at that point to run your brush all the way to the center and still provide enough cleaning effectiveness.
However, there also appears to be significant mildew or algae growth which if you have a pressure washer and some detergent, I would use that to clean that gunk off first and use your WFP to remove the detergents and any residue from the glass surface afterward.
Don’t know if you have already done this or not, but I would talk the client into at least a semi-annual cleaning if not quarterly, just so that it never gets this bad again.