The problem is that modern brown malt is nothing like the brown malt available to the ~1800 porter brewers.
Brown malt ca. 1800 was a grade of base malt. Though it had been kilned to a high color and flavor, it still had the ability to convert itself, as you know. Modern amber and brown malts are roasted malts which have had all the diastatic power destroyed. Moreover, they're usually made in a drum roaster based on the same design that permitted the manufacture of Black Patent malt, which revolutionized porter brewing!
I don't think it's possible to even approximate all-brown-malt Porter. Believe me, I've tried; I haven't yet stumbled upon an ingredient or technique which approximates the malt to my satisfaction. Other than, of course, using a mix of malts and calling it a day!
It's just one of those things we'll never really know about until DARPA gets off their asses and funds time travel.
Cheers,
Bob