Yep - all you have to do is change the details slightly when you tell them that you're on a special diet. As you discovered, everyone assumes low fat. You just have to be more specific: you're borderline diabetic (or PCOS, or whatever health reason you don't mind sharing) and have to be careful about sugar or starch. If you tell them this ahead of time, even if you tell them not to go out of their way, they can't complain if you don't eat what they cook.

Anyone who gets upset with you just because you won't eat their stupid cheesecake has *serious issues*!!! Nobody has the right to force-feed you for etiquette's sake.

Of course, as you probably already know, you can offer to bring something you *can* eat: a cobb or caesar salad, an appetizer plate with lots of LC stuff, a LC dessert...

Hope things work out and you have a nice evening with her/them.