Anyone have any experience/advice on painting Dan Henrys? Do you use a stencil or freehand? How do you spray paint that much without getting the paint all over yourself, your vehicle, etc.?

Use a template mounted to a wooden stick.
Put down the template and spray with paint (lightly).
Should not get much paint 'all over you' that way.
You can also wear pair of rubber gloves. gloves.

Also there are special line marking cans of paint out there that are designed to work upside down that don't overspray too much.

I make them freehand with the inverted marking paint spray cans.

Bring a large plastic bag to put the stencil in; it gets covered in wet paint. While slightly more expensive, our city and county require us to use a "paint" that is really just chalk in solvent. It washes off after a few rainstorms. It can also be easily scrubbed off with water and a brush for those inevitable mistakes.

When we do a wildcat ride, we just freehand the markings. It only takes a bit of practice to be able to make decent arrows and three-letter words.

One more piece of advice: do the markings by bicycle rather than by car. This helps you see those many places where you need one extra marking well in advance of the turn. Also, assume most riders are riding faster than you and are less attentive and mark the course accordingly.

Any photos or models for the stencils mounted to a stick?

Actually. Dan Henry himself wrote some quite detailed instructions.

He recommended marking the route by bicycle. He used a brush and kept the paint in a water bottle (I think) under the down tube. No stencil and he was quite specific that a simple circle with a line indicating the direction of travel was best. Keep in mind that these directions were written in the 60's so they're a half century old.

My only other advice is that, if you ask prior permission, some places are going to tell you "No."

I disagree with this. STP (Seattle to Portland) uses this method and I found myself having a bit of trouble determining the direction of the arrow when I rode over them at speed. These are particularly bad in low light conditions, which is what I was dealing with since we weren't actually riding STP, just using the route the day/night prior to the event on our way home from Seattle. I prefer a simple arrow with the tail long enough (about a foot) to see at speed. The Davis Double Century used to use very tiny arrows, less than six inches long, and the only thing that kept most folks on course was the presence of nearly 2000 riders who knew the course by heart.