I'm conflicted about this Tracy. On one hand I was raised by parents who wanted me to have good manners. Their general expectation was that if I was invited for a meal that I be polite and at least try to eat what was put in front of me. At least taste it.
My take on this is that if the meal was inedible (forgive me, I don't know what boiled dinner is, I'm from the south and I don't think we have that. Is this stew? Stew is usually OK, not my favorite but I can't imagine how stew would be rejected and McDonald's preferred.) I would push it around on the plate and give it a go and then if the hostess made notice of this it would be on her. Bad manners to question what a guest does with the food on their plate.
If pressed I would claim not to be feeling well. "I'm sure it's delicious! I just don't feel well tonight." Then ask husband to take me to McDonald's (or maybe someplace much better!) after we left.
On the other hand, I've raised a kid. I always asked her to try new foods once a year. Kids tastes change. I'm a good cook so if she didn't like it I chalked it up to her not enjoying the flavor of whatever it is she refused to eat. I asked her to be polite when invited for a meal she didn't care for. You really can make a plate of food look like you've given it a go. Easier if there is a dog handy.
I hate the idea of eating gross food building your character. That sounds completely crazy. Taking a taste seems respectful though.
This boiled dinner you speak of sounds horrible...worse than the "most vindictive flu"!
I would not want you to eat food that was unpalatable. I also wouldn't want you to be rude to a person as important as your spouses mother.
Having good manners often involves a lot of lying, sometimes means you have to be creative when facing hardships. You also sometimes have to take into account the starving children of (insert favorite impoverished nation or area) .
Maybe after you explain boiled dinner I'll have a different take on things.