I seem to recall seeing an RAF FOB once which required that Muslim pilots were not allowed to fly as pilot in command after midday during Ramadan. Whether this was a local procedure following a specific case, or a general RAF rule, I honestly don't know.
I'm not a religious man, so there may be reasons why not. But, I'd have thought another approach might be for Muslim aircrew simply to be scheduled for any ground training, sim work,
PR stint etc. that is not safety critical during that month - wouldn't take an organisational genius to manage this without significantly mucking up organisational efficiency.
G