"Try to set up a sprinkler on the roof and just leave it... it'll go a long way to knock down any embers that may land on it."
This is a fantastic idea and something we have begun to implement this year for our house and one of our barns.
I've been meaning to do it for the last five years but it's more complicated than you think - your water supply and all components out to the sprinkler heads need to be metal (not PVC) pipe and depending on the height of your buildings it might be non-trivial to get the right pressure up to the top of a 30 or 40 foot tall structure.
I cannot comment on issues of "neighborhood water supply", etc., as we have our own water supply.
The plan is very simple - upon evacuation, turn on the sprinklers which will irrigate the area of the roof itself, as well as 10-12 feet beyond. It's a very good idea because much of the risk of structure involvement arises from embers that are blown, or float onto the roof, into the gutters, or into the attic (through vents).
Related to that, it is extremely important to keep your gutters clear of leaf debris and clear any accumulated piles of brush away from the house itself (usually in exterior corners, etc.)
For last minute option, perhaps best to attempt to make a sprinkler out of the gardenhose by poking holes in it and if long enough roll it out around the house.
I know you're joking but California is a whole lot more than Southern California. California was on my shortlist of places to retire to and a requirement to be on that list was lots of snow.
http://www.readyforwildfire.org/Pre-Evacuation-Preparation/