I would also like the option with the grouped meshes 
I like the idea of having artist control over it and yes…it will be extra work, but it also allows for a lot of freedom. Automatic tools are not always leading to good results and yeah…I personally prefer a bit of fine control Also, it sounds kind of weird to combine everything into one group anyways, so I wouldnt fear that to much. Or maybe make a special group that is called lighting group or so. It doesnt actually group the objects, it only groups them for lightmass. You could also have a visualization mode for this so stuff can be easily debugged.
just my 2 cents^^…cheers!