Nerd,
You are correct about Leadbelly and overalls, in the film for Pete Seeger anyway and there was that awful reconstruction on the March of Dimes programme where he dressed in prison stripes.However I'm sure that left to his own devices he would always prefer to wear a sharp suit.
Broonzy however was coming at it from a slightly different angle having been a successful entertainer on the Chicago club scene and playing electric guitar in live performances there. When he came to Europe he was back to playing an acoustic (Martin 00028 herringbone for all you guitar freaks) instrument but still wearing sharp suits and was still a wonderful entertainer, story teller, singer, instrumentalist.

Greg, regarding Bill's fiddle playing I really like what little he did but however much I appreciate Bill he was after all only one of a very large number of musicians around the scene and I don't personally think that it would have had that much effect on the general development of the music if he'd stuck to the fiddle.
As a matter of interest Lonnie Johnson of course also played fiddle AND piano, Lightnin' Hopkins played piano and on one recorded track organ, Brownie McGhee also played piano. It's what the record companies chose to put on wax that has given us a skewed view of what was available.

Finally to get back to the origins of this thread Leadbelly, he also played piano in a primitive rocking style and if my memory is correct also used a six string guitar on accasion