powdercoating spokes is probably not a good idea.
From bicycle wheel building experience, you want the spokes to be able to flex just a little both in rim torsion and impact. A hard coating would probably crack and chip at one point or another.
Also, an extra thick spoke would be harder to lace too. I know I would probably damage the spokes coating while futzing with the wheel build.
Personally, I would just try rattle-canning your spokes with good spray paint (Appliance epoxy works awesome!) and seeing how it looks. At least with cheap paint you could always just touch up the paint later.
Some fixed gear freaks like to color their spokes with long thin strips of colored electrical tape. Not the best for weight- but it holds up really well.
Best of luck!