If you turn on Soft Shadows in the Premium render setting you'll get IBL SS but only a small amount. I'd say it's about the regular lighting equivalent of setting SS to about 5-10%. Needles to say it really slows down the render.

You may be better off upping the IBL quality. The guy that did the IBL algorithm that Bryce uses recommends a minimum of 512 lights (the Quality setting in Bryce) with 1024 being preferable.