Landscaping contractors do this in our prairie city here in Canada. They drive slowly by the boulevard trees watering the curb and the street too, while the driver reads a magazine or stares at their iPhone. /s As another drought "feature" our City council voted last year to put in residential water restrictions telling HO's what days and what hours they can water their own trees. Fines.
If you do have water restrictions, upper scale neighbourhoods and a source of surface water (maybe you'll need a license for this) it could be another revenue stream. I might look at churches with large yards and trees too or anyplace where there isn't a caretaker or landscape service already. To make money though it'd probably have to be minimal time spent at each tree so some sort of flood setup rather than slow trickle like sweat irrigation hose under mulch layer. My 2 cents for this morning. Good luck.
