Depends on the design of the lens, for example, the older "convertable" symars can be used with only the front or the rear group, but the focal length is around double that of the complete lens. So, if this were true of your Bronica lens, (and it likely is not) you'd need a fairly long extension tube to get the lens to a place it could be focused.
If you're just looking to make things soft, you could possibly unscrew the element a few turns, some of the soft focus lenses work in exactly this way. If you're looking to get vignetting, a deep lenshood might work well.