Yes, not only a slug, but also pieces from castings that weren't of good quality. Just break or cut then into "Bite size pieces" and put them in the mold's cavity. JMHO, but I believe you could also do that with any, hyper buoyant, non porous, strong material, i.e. light weight PVC. From the outside, the casting looked like any other 100% resin lure.
Here's a spook with a wood dowel core that is nearly the six=ze of the whole lure. It will be a Bluefish lure, so longevity isn't an issue.