Is this a rude way to make sure our wedding guests eat instead of just getting drunk?

I think your idea is a very delicate way of getting around such an awkward situation. I wouldn't be able to suggest a better way of doing it, and I know it must be such a horrible thing to be looking forward to so I sympathise! I think that using money as an excuse is good for now, and then once the wedding is over you can either casually avoid those members of the family and let your fiance do those visits alone, or you can gently bring it up with him and force him to acknowledge it. That conversation doesn't have to turn into a fight, and so him getting defensive and arguing means he knows there is an issue - so just be gentle and say you want to support him, not be against him.