The rules say "no blinkers" for xc, but I don't think a fly mask would be interpreted as blinkers. I would hate to drive 10 hours to an event just to be at the mercy of the TD's interpretation each time.
I wish I was handy enough to make the horse some really fly sunglasses! He has enlarges corpus nigrus (I spelled that wrong), that blocks/reduces his eye sight when it's bright out.
I think that the fly masks designed for riding in would be fine. At AEC's last year the winner of the Novice Amateur division Cady O'Daly Michael went around in one of the Cashel black fly masks. You might need to get the OK from the TD but I don't think it is forbidden.