Hi Chris, it’s certainly been done as I’ve saw some pictures on a couple of boats up for sale some years ago.
From memory, it looked like the anchor locker lid had been cut in order to mount the windlass, presumably on a support strut. I think the D30 Classic Facebook group might be helpful….seem to think there’s at least one with a windlass from photos posted. My enquiry on here 4/5 years ago regrettably drew a blank, there are only a few 30owners. Good luck. John (Ex-Calandra D30C)