Unique and Blue Dot have been used in nitro for black loads in shot & ball and other shotguns. Since you have nitro proofs, you should be extra fine. The Blue Dot load Fosbery loading that worked in my paradox type generated 2 3/4 tons pressure (pressure tested by Armbrust). Think I would try Unique first though as Sherman Bell has pretty much standardized the use of that powder.

I pattern tested a few shot loads, mainly the gamebore from Kent as I wanted fibre wads. I found my normal game gun 28 gram loads crossed from the heavy paradox barrels. I needed to go to the heavier 30 gram pure gold gamebore loadings to get the barrels moving enough to superimpose the patterns. Velocity & recoil have to be just right. These should be very close to the commonly proved for 1 1/8 oz. 3 TPSI standard English shell.