Rob reminded me that I usually hid the probiotic in a bit of banana. Banana should only be fed sparingly to guinea pigs, but it is a wonderful medium for hiding pills and powders. It worked great for Willow's shilintong... until she realized that banana shouldn't be crunchy. She was very deft at eating all the banana and spitting out the shilintong. Bad pig!
However, Pinni isn't so discerning. Probiotic powder wedged between two thin slices of banana quickly disappears. I'm not sure if the banana flavor overwhelms the probiotic, or if it's because she eats it too quickly to notice. Doesn't matter - it works for me!