-bgfx_screen_chains takes a list of .json files to apply to each screen (for a multi-screen game). But that seems only tangentially related to what RC wants to do, which is not actually possible since JD removed the BGFX parameters from the ini file.
Since I'm using hlsl in the bgfx_screen_chains it actually has both scanline strenght and shadow mask strenght, which are the different params.
I will try to do what QMC2 said, to create new .json files and see if it works.
EDIT: It works! Thanks people