I was thinking of using a pressure cooker to prepare the Flemish rabbit. This requires stewing the rabbit legs in beer.
Is it safe?
It is perfectly safe to add beer to any recipe in a pressure cooker. (well, any recipe you might use beer for outside a pressure cooker). While I've not done this with rabbit I have with venison (for chili), beef and chicken for various stews.
Assuming your concern is adding a 'carbonated' beverage, this question has been answered here: Carbonated Beverage in Pressure Cooker