it is rude to refuse refreshment and food when offered. you should accept the hospitality even though you know that you will not sit through the whole dinner like in a wedding banquet. just take the seat and have a short rest, drink and a few nibbles. refusing your host's offer will make the situation awkward for everyone.
but ALWAYS refrain from alcoholic drinks. you can drink to your heart's content after the event if you are offered. or maybe challenge the groom to a jug or two.
if it has been overlooked and you are hungry because your event coverage cross over meal times, just ask politely for time off to grab some food on site or off site. most likely the organiser will arrange something for you.