I Googled around a bit for info on this. Thanks for mentioning it.
Didn't find anything exactly like you describe, but some folks have tried this with regular hollow points. And while it does increase penetration, it doesn't do much for expansion. The "hydra" in "hydra shok" means "hydraulic," so it seems the insert must be softer than the lead, not harder (so it can flow outward and aid expansion).
There's also some concern that it may be legally considered "armor piercing" since the BB basically gives the bullet a steel tip.
Also found one guy that had filled regular HP (in various calibers) with silicone caulk, and said it performed very much like Hornady's critical defense ammo across the board, and that it keeps the HP cup from scooping up debris from layers of clothing. He actually used old Wrangler jeans (several layers) for testing this.
A very interesting subject IMO.
