Okay, I see...
While the Zoerk is a fine system, it's not close to what you're after. The Vivitar isn't quality enough to consider mounting before an L lens, and I'm not familiar with any medium format equipment thats going to be helpful in such a situation.
Now, for the original thoughts of a bellows... one will be even more restrictive than a lens-less extenision, here's why:
1. They only compress so much, so it wouldn't offer any advantage over an extension tube.
2. It's going to be more flimsy, and can't support the 5.6 lbs of your 300mm as well as a tube.
3. Actually, bellows I've used are generally thicker at full compression than a #3 tube, so would be more limiting in long focus because of it.
What you're really needing is a reasonably sharp lens, with close focus ability. As an alternative to an auto bellows, you might consider an old 100-300 f/5.6L which has "close focusing". Information can be found in the on-line Canon Camera Museum:
http://www.canon.com/camera-museum/came ... index.htmlOf course, the easiest option is to change lenses, and not be too concerned with dust. Just purchase some pads to clean your sensor after each outing instead.
I know that isn't what you want to hear, and I understand 8.2 feet minimum focus is restricting. Mark it up as the price for having the sharpest lens you can get, and it's limitations are required for it's strength.
If you find something which does what you're wanting, I'd love to hear about it.
Thanks, you've got me thinking. Just wish I had a better suggestion.