Well, try it, but my guess knowing how it works, is that you can't control how far into the tube you fill without a plug of some sort, and it will definitely effect spine. It has a pretty rigid set, even though it is extremely light for it's density. Now if you had a way to control how far you filled, and could predict the effects of stiffness, you could in theory buy under spined arrows, and add the foam to gain a proper spine and weight. Sounds like you will be doing a lot of experimenting, or talking to the manufacturer of the product and more over the little guy in the dark back room with the pocket protector. They may can provide you with a mathmatical basis to predict how the substance will act. They would probably hit hard though, kinda like a corked bat!
I am curious now, let us know how it works.