Generally speaking, with maple/maple necks, whatever finish you apply to the back of the neck is what you would apply to the fretboard. Most commonly used are polyurethane, Tru Oil (which contain polyurethane) or lacquer (either acrylic or nitro). It's just personal preference.
I'm sure it's been done, but I've never heard of anyone using two different finish materials for front and back. Potential there for incompatibility issues would be one reason not to.