I can gets these really cheap locally at the food co-op and I cannot get rice hulls (LHBS doesn't and won't stock them).
They would seem physically to do the job nicely in keeping the grain bed from compacting. My only concern is whether or not they will add a flavor dimension as buckwheat itself is a rather assertive grain, well, okay fruit actually. I think I will try steeping some alone in mash temp water and see if it makes any noticeable flavor/aroma but it would be good to hear if anyone else has had any experience with it.
If I do use it it will be in an oatmeal stout mash so it may not really matter. Might add a complimentary dimension to the roastiness. Who knows.