With a bore slugging to .512", there will be no problem shooting .515" bullets as the softish lead will easily swage down without causing any great difference in pressure. Again, you are using lead, which is very malleable.
As a rule I have usually sized bullets to .002 - .003" over bore size as a matter of course, with no problems encountered in either antique or modern made firearms.
With regard to reduced loads of smokeless to duplicate old black-powder loadings, it's either "No problem with that" or "Never even think about it !!" It's a personal choice.
I've never owned/fired anything in 577-500 No.2, but have owned and used quite a few antique and modern rifles in 45-70 and several other calibres, using both black and smokeless powder loads.
I have found 28gr. of IMR 4198 (40%) to duplicate original 45-70 load performance with 405gr. bullets, as well as being accurate and comfortable to shoot. I still use this load in a little, light weight 45-70 H&R Handi-rifle just because of those reasons. NOTE that this is a straight walled case, NOT a bottleneck like your .577-500 No.2.
With regards to felt wads and dacron fillers, again it's "Yes" or "No".
I quit using any wad or dacron with smokeless for fear of a 'ringed' chamber, and find that 4198 burns well, leaving a few traces of un/partially-burnt powder in the bore, but very little.
AGAIN note that I'm talking about straight walled cases and NOT a bottleneck like your .577-500 No.2.
You might want to go to the British Militaria forums to ask about your rifle, as there is a lot of knowledge there re. older Brit. rifles and sourcing needed loading items and recipes.
You might also want to go to the Cast Boolits forums for excellent general casting information, as well as discussions re. wads and fillers with smokeless loads.
I also see that 'Cartridges of the World' shows 37gr. 4198 with a 360gr. bullet for .577/500 No.2 Express.
The 'Handloaders Manual of Cartridge Conversions' also show the same load, with a notation that it is a black powder cartridge from around 1883, but then references it as a 'Nitro Express'.
They both also differentiate this cartridge from the later (1905) and longer .577/500 Magnum Nitro Express.
Lastly, once you have everything set up to try, you might find that you need Lyman's mould # 515139, which lists as 340gr., and might be easier to regulate to suit your rifle.
Boy, it's a can of worms, but a really neat project. Good luck !!
Regards, DaveF.