The concept of using nitro equivalent in BPE doubles (Nitro for Black) is is not new and not American in origin. The British began offering smokeless shells (loaded with Cordite) for Black Powder Express rifles in the early 1890s and Kynoch offered them well into the 1950s.

From the 1910-11 Eley Brothers catalog under "Nitro Cartridges for Black Powder Rifles":

"These cartridges are loaded with Rifle Cordite and metal based bullets to give at 60 degress F slightly higher velocities than the Standard Black Powder Cartridges. They are perfectly safe in rifles of sound construction built for Black Powder, and are distinguished from the High Power Cordite Cartridges (which must on no account be fired in a black powder rifle) by the Lead Bullet with Metal Base. Smokeless charges giving pressures higher than black will not be used in Express Cases in conjunction with a Lead Bullet." -- In other words, smokeless shells loaded to black powder pressures specifically intended for Black Powder Express rifles - NOT "special" Nitro for Black rifles. Nitro for Black is black powder proof. The peak pressure required of a Nitro for Black "purple stripe" proof load was the same as for standard black because the standard pressure was the same.

The standard black load listed for the .577 2 3/4" Express was 160 grains black and a 520 grain bullet. The smokeless load offered was 73 grains Cordite and a 570 grain bullet.

For those who didn't see it, Sherman Bell's article "Finding Out for Myself, Part VII, Express Rifle Pressure Variables" in the Spring 2004 DGJ discusses the Black vs. Nitro for Black issue. A Henry .450 BPE with damascus barrels was used to develop loads that would regulate in the rifle and those loads were then pressure tested. A standard load of 120 grains of KIK ffg black and a 300 grain cast bullet gave a velocity of 1812 fps with an average pressure of 21,600 psi. The current Nitro for Black equivalent formula developed by Ross Seyfried (40% of black in IMR 4198) of 48 grains IMR 4198 with the same bullet gave 1952 fps and an average pressure of 21,700 psi. Pressure curves were similar. The black shot into 4" at 100 yards and the Nitro into 2 1/2". The Eley Bros. smokeless loading for the .450 BPE was 55 grains Cordite and a 270 grain bullet and Kynoch loaded 52 grains Cordite with both 325 grain and 365 grain bullets. The pressure rating for these was the same as standard black, 11 tpsi. For comparison the .450 Nitro Express, which uses the same case, was loaded with 70 grains Cordite and a 480 grain nickel jacketed bullet for 17tpsi.

Sounds like Seyfried and Bell have discovered the same thing the British did 113 years ago. The safety issue with these rifles is the peak pressure and the pressure curve, not the propellant used to produce it.
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