Hi 9.3x64mm
A couple of observations:
1) Damascus barrels are NOT a deal breaker for shooting proper Nitro for Black loads in a rifle in good condition.
2) Duplex loads for BP are easy.....~10% Accurate 5744 under a commensurate amount of BP. Weight of BP reduced by 3x the weight of smokeless. ie: in a .500 3" (nominally 136 grains of black) you would use roughly 13 grains of 5744 then use 95-100 grains of black. As with straight Black Powder do not tolerate any air space between powder and bullet base use wads or lube cookies as necessary.
3) a safe Nitro load for most any BP Express would be 40-44% of the original BP load (by weight) using H4198, magnum primer, and either packed in Dacron filler, foam backer rod, or the foam wads from Kynoch.
4) Graeme Wright does have Varget used in some of his loads but there is, as you note, no mention of some more obscure calibers like the .500 No.2. Not that it could not be figured out......

Easiest course of action for a "Nitro for Black" BP Express double rifle to start is look up the old NfB load and use that bullet weight paired with % H4198 of the original BP load as above. Then work from there.
Often the load of H4198 is very close to the original NfB load of Cordite.

Pretty much all this info can be found in 3rd edition of Graeme Wright's book.
Make sure your mate understands that these are not to be expected to be loaded to exceed the old specs.

Note: I have a William Richards, Liverpool .500 No.2 and love it.
- Mike