It makes sense to have Hardwick as a guest, and I can't believe he's too busy to fit it into his schedule. If he is playing DnD with other people, he can certainly make time to drop by Nerd Poker. Of course, it is also possible (since @midnight has like 9000 writer) that the entire DnD joke was a written set-up specifically for the game they were about to play.
Either way, Hardwick would be a welcome guest in my opinion.