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?

SystemThree produces colors ground into epoxy resin - very concentrated colors, including white.
http://www.systemthree.com/store/pc/Epoxy-Paste-Pigments-c46.htm
I've used these with WEST epoxy with no bad results - the amount of color paste you add to a typical epoxy batch is a fraction of a teaspoon, so you don't need much additional hardener to compensate! On a boat, I've used these colors mainly to make scratches through the finish less obvious - I've always top-coated with primer and paint, so I wouldn't count on getting a full-coverage coat from just the epoxy paste.
For a paint base, I'd add a fairing compound coat (thin- Microlight) and then a primer-surfacer (Interlux and some others can be brushed/rolled- even auto finishes can be brushed with the right reducer) to give your artist a consistent working surface.

BTW, I used white zinc oxide (and silica) in epoxy years ago to make the interior of an icebox, and it worked well.

Also, I am thinking of graphite in the mix on the very bottom. I understand that it won't have any practical effect in terms of "slipperiness", but I hear some add it for "hardness" and scratch resistance for rock play. Does it actually help in this regard? What are good sources of graphite powder for this, and how much graphite powder to include in the epoxy fill coats?

This is a topic with LOTS of past comments....One common mix includes graphite, silica (Cab-O-Sil aka fumed silica), and epoxy, and my opinion is that it is the silica which gives the abrasion resistance.