It depends on the jacket. I personally would not go a size up on an Aero Thirties Halfbelt, or similar pattern. Such jacket were designed and cut to be worn neat, and to my eye they look over-sized and 'wrong' when worn big to fit in a sweater. An Aero Fifties halfbelt or something along those lines, with a more generous cut, might work. Personally, for something to go over a sweater, I'd be looking at something that is boxier by design, like a Highwayman.
Another thing to consider... when are you likely to wear this? By the time it is cold enough to require a sweater in addition to the jacket, will you really be all that likely to be reaching for a short leather jacket, or will you always by preference reach for the long, wool overcoat? Versatility is all very well, but if you compromise on something you really want in order to make it more versatile, then never use that flexibility, there seems little point to me. All comes down to what you really want.
I respectfully disagree with that. I don't think jackets look great when they are so tight that you can't fit a few millimetres of fine wool sweater and shirt underneath them. I have sized up my 1930's half belt and I do not regret it. Fit's better/looks better. Not in the slightest baggy. I don't work out ever but I should add that I'm 6 feet 2 and 185 pounds with very little body fat. In other words I did not size up to accommodate a beer gut.