Yeah, they won't get offended if you tell them you mean the real spicy. Especially if you are/become a regular.
They're just afraid of some cac making a mistake then bitching about it, for similar reasons, that's why the back half of the menus are never in English to stop some cac from ordering off it not knowing it's actual Chinese food, not Chinese-American food.
Once they know you're cool they won't give you any trouble. "On the side" is a good way to ease your way in.
I do agree with porkbun that black places will be more leery of this than immigrant places though. There was a creole place in the city a decade or so ago that me and another cac friend would go to, they only made two dishes a day so there were no real choices outside of that and we'd always get stares from the customers when we went in until the woman started saying "hi boys" and signaling to them it was okay when we came in.