The only simulator in which you could add custom operator was P3D 4, because there we had access to Render to Texture and our custom implementation using DX11, this was already abandoned in P3D V5, in which we reverted back to the standard multi-liveries, because even if we could have Render to Texture there, it requires DX12, and we simply lack features we needed for the DX11 version, so even P3D V5, it's no longer possible to create custom textures.
MSFS doesn't have any form of Render to Texture ( programmatically load and display textures on the fly ), and when asked about this, both Microsoft and Asobo said it's not a planned feature, for performance reasons.
