hot chocolate stand?

Can you ask your venue if they'd do it for you? Maybe you could add something to the bottom of menu cards, have a sign up on the sweetie buffett/evening buffett/ at the bar? maybe they could give you a small bill for it the following day?

With one company I work with, at meetings they always have hot coffee milkshakes.

Its very simple its like a large tea urn, but the only difference is that the element is under the container, not in it like a lot of kettles.

Otherwise it would burn the milk.

I'm sure you can hire one of those easy, and I cannot see any reason it wouldn't work just as well with chocolate.