Are full art promo cards legal in tournaments?

Yes, if they are the official ones from FFG.

Realistically, it's also unlikely that a TO would stop you playing with unofficial alt art cards for characters, as long as they are correct and easily recognizable.

Deck cards are likely another story, as they could potentially be different enough from real cards to be considered marked cards.

8 hours ago, Abyss said:

Realistically, it's also unlikely that a TO would stop you playing with unofficial alt art cards for characters, as long as they are correct and easily recognizable.

Especially since you had to pay for the dice anyway!