The deck of my next boat will be hand-painted by an artist friend, so it needs a white "primer" for her to start. This will be on top of a layer of fiberglass. Is there a white pigment powder I can add to my fill coats of epoxy to create a white finish? (instead of an actual layer of white primer?

If the artwork is going to cover a majority of the deck then pigment could work. West System makes a liquid white pigment that works quite well. If the some of the white is going to show then why not just use primer/paint and then clear-coat it once the artwork is done? The paint will cover much quicker and with fewer layers. It'll also be more even than trying to tint the epoxy. With epoxy you'll need more than just the regular fill coats to get it totally opaque.