"Anyone tried to CNC cut an entire Styrofoam hull plug, epoxy/glass the outside and then dissolve the Styrofoam with a solvent? Do you think it would work?"

I did it with a 12' kayak...shaped a Styrofoam hull and glassed it then mechanically removed the core with the help of a HD Shop Vac. Dissolving it was too messy in my minds view. Originally I was going to leave the core in but with a shape of any real volume the core is too heavy.