For a BP-proved 12-bore ULH, I would consider a fully-rifled 20ga bore-gun. Barrels are available from E.R.Shaw, Hastings, etc, and a mold can easily be made for either the Fosbery-style projectile or a heavier lead slug. Bore-gun pressures for the smaller 20ga could be kept well within the BP 12ga range.

You don't give any indication of your location, John. If you update your profile, we would know whether you intend to annoy the local white-tails, or something bigger...!