I have used airbrush thinner to thin paints for hairy brushing before, it's not terrible but the airbrush thinner is usually a stronger solvent than is good for hairy brushing which can make it tricky to use.
Personally if I'm trying to get super smooth coats I thin with mixes of water, flow improver, lahmian medium and glaze medium. The flow improver breaks the surface tension and lets the paint spread easier, the glaze medium makes the paint dry a bit slower which further helps levelling and also acts like a bit of a binder so if going super thin it stops the paint separating out.
I don't use a wet palette though, just my dry palette. I could imagine flow improver MAYBE causing problems on a wet palette.