I'm a bit late poking my nose in here, but could you just add some psyllium husks or something to the shakes, to bulk them up with fibre?
As an aside, my husband likes sugar-free aloe juice for pooping purposes. It lists a lot of carbs but they're nearly completely fibre. I think it's only 5 calories per cup or something like that. Tastes good, too, although I'm not much into drinking it myself (because I'm avoiding artificial sweeteners).