Go out and buy some Theakston Old Peculier for an example of a beer with Molasses character.
It imparts a funny, complex, rummy/winey type character, which IMHO goes very well indeed in a malty dark ale and would probably work in a porter or stout.
You don't need to use much to get that effect - a tablespoon in a 23 litre batch is very noticeable. If you used 200g I doubt it would be drinkable (for my tastes).
cheers,
Colin