How about....flying buttresses? They worked great for hundreds of years in the real world.
Sorry - I just couldn't resist that one.
I've yet to design any parts that need support material, but I I did I'd try to find a way to incorporate the required support into the overall design of the part. In the example given by the OP, how difficult would it be to add some thin triangles to support the roof, and then take them out by hand when printing is complete?
Of course this would not work for a pre-done/downloaded part, so for that situation the option to have support like the OP suggested generated would be a nice feature.