I like the idea of a preset menu, and then telling people about this and what the menu is beforehand. That will make everything clear so people have full information on which to base their decision. For dietary restrictions, I do agree that you could end up choosing things that one particular person can't eat; but at least they will know about it ahead of time, and they can make alternate arrangements.
I have dietary restrictions now, which are a bit vague. Generally low-fat but there are a lot of components for me to think about--it isn't just, "chicken instead of beef" or "always a salad." I don't expect anyone else to be able to figure out how to accommodate me when I can hardly even tell them myself. But, if I researched a restaurant's full menu ahead of time and found that I was comfortable with entree X, only to get there and find a preset menu without entree X, I would be stuck. At someone's home with a casual atmosphere where people are grazing buffet-style, it's easier to get away with not eating anything, or just having a bit of fruit. At a restaurant, sitting at a table, it's painfully obvious if someone has decided to decline eating anything. At least if I was told beforehand of the preset menu and what its options were, I could decide whether to attend or not.
Also, it depends on if your particular relatives have displayed gimme pig tendencies before. If they haven't then I wouldn't worry about it showing up now. And if one person does anyway, hey, valuable information for the future.