I've consulted on after the fact irrigation jobs that were screwed up detailing every thing in the design or lack there of that contributed to the failure to get irrigated properly. Usually involved a lawsuit or threat of one. $65/hr, 3 hr minimum which includes site visit, travel and report prep time.
The idea probably has more merit like a Dr making house calls than as a construction supervisor. Maybe $75 min for 1/2 hr on site, they take notes, you document yourself for your own records. Talk about turf and ornamental insects, diseases, cultural practices etc., whatever is on their mind. You just need to have some certifications, degree(s) experience etc and really be on top of your game. Priced high enough it could fill a few blank hours a month an make a few hundred with out busting butt.