I don't eat eggs. I'm not allergic to them, but the taste, the texture, the smell all turn my stomach. Eggs are pretty much the only food I can't choke down in the name of politeness. I know it's all in my head, but I feel nauseated all day if I eat the least bit of egg.
So if I saw my hostess in the process of making tuna salad and there were eggs involved, I'd ask nicely if she could put aside a portion for me without the eggs. In the case of a lunch of tuna subs, there probably wouldn't be much else in the meal, so I wouldn't be able to fill up on sides. And there's no way I could eat tuna salad with egg in it.
If it were something like mushrooms, which I don't like, but I can eat without problems, I'd keep my mouth shut and just eat the mushrooms. And I prefer Miracle Whip to mayo, but I'd never mention that, either. If I kept my mouth shut about the egg in the tuna salad, I'd have to come up with an excuse as to why I wasn't eating the main part of the meal, but filling up on carrot sticks (or whatever else was on the table) instead. Would a host want to prepare a meal for a guest and then not have the guest eat it?
But I wouldn't expect more tuna in my sandwich. I'd expect the host to remove a portion of the tuna, say 1/4 if there are four people, and mix that with whatever else was being added and use that for my sandwich. If that meant I had a lot of mayo and relish, then I'd eat a lot of mayo and relish along with the tuna.