I'll occasionally wear tweed jackets in summer. It's not really that bad, even in the kind of heat we have down here in Texas.

Remember, there was a time when men wore waistcoats and jackets all year round, regardless of the heat. And that was before air conditioning! It won't kill you. It just takes some getting used to. And in this day and age where most buildings have air conditioning, the type of events you're describing that you'd wear the jacket/waistcoat to shouldn't be that bad.