This would surely hugely depend on where you are based, no? There used to be a van that drove around London doing bike cleaning but I think they went out of business - at least I haven't seen them around recently. There is however a mobile bike cleaning business in London that use some cargo bikes - a google will find them. A bizz would probably flourish in areas with no gardens, right?
I was thinking of starting a bike cleaning business myself, for no better reason that it would allow me to invest in a karcher. Now, I know what you are all thinking: A karcher? That will ruin your bike! Well, Seth of Seth's bike hacks did an experiment where he found that it didn't do so. Besides, even if you stuck to using the karcher on the parts of the frame away from any components, I think that alone would save you a huge amount of time. I've spent hours training my hose up and down a single bit of frame, with the road grime just refusing to budge. But in any case I never did set up the business, never did invest in the Karcher, so for all those who want to yell at me for even suggesting it, no need.