I believe Barbados is a carbonate platform environment?
I'm really rusty on my geology, but the clay may have some carbonates in it, and may give your hardness and pH a bump (not necessarily). If you are using it as part of a dirted tank, or under a cap, it should be less noticeable.
heres something I ran across with a listing for composition of barbados clays (hopefully the link works...):
Those samples have very little carbonates or salts, so odds are it won't do much to pH or hardness.
So, yeah, if you can get some failed pottery, throw it in a sack, smash it up to gravel size or smaller, and give it a try. Let us know how it works out.