Neat idea Ian and good to see some thought has gone into it even if it is a bit overkill. I think its back to dunk in bucket for me or i might try the back pack. The whole point of my first suggestion is saving time and it could almost be even between those two when you factor in filling backpack, taking out of van (charging once in a while too) compared to TFR in bucket. Just a shame the inline dispenser won't work very well. Oh well back to the drawing board 
it doesnt get much easier than simply applying your chosen chemical to your brush head using a spray bottle.even filling the backpack with a chemical mix doesnt take long(esp if you do it the night before)and plug your main pole in and apply to filthy f/s/g then swap back to pure.
if you price well for add on jobs an extra couple of mins spent applying chemicals doesnt bother you.
i did a yearly conny roof clean this morning and used a 5l pressure sprayer that already had a strong virosol mix in.a couple of sprays on the roof.dead easy and quick.i was all done after 90 mins (from pulling up to driving away.)