Randy- I thought oil based on top of water based was a big no-no.
Won't it peel and sluff off? Unless it's water based primer underneath? Also, oil based is a nasty medium to be working with from a wheelchair. It gets all over everything and has to be removed with paint thinner. I'm picturing a major paint containment problem!
I defer to you since it sounds like you know professionals. 
I originally thought that as well. But I have seen panels painted in this manner hold up to 5 years of
outdoor conditions and they still have no problems.
Latex, being as thick as it is (BTW, do what the label on the can tells you and NEVER thin it), does a pretty good job of sealing a porous surface. Once dry, it's compatible with enamels, but I would stay away from lacquer or other aromatic solvent based coatings.
As for the mess, latex, once dried, is no picnic either. Just because it's water based doesn't mean it comes off any easier. It just means cleanup can be done with water instead of mineral spirits.
For what it's worth, dried enamel can often be removed with mineral spirits, whereas dried latex can't, and water won't do much to it either. At least that has been my experience, YMMV.

RandyT