If you set bgfx_screen_chains option to crt-geom in mame.ini then it should stick as long as you're running -video bgfx. If you're running video d3d then that setting isn't used.
In your MAME\bgfx\chains folder, if you open crt-geom.json or deluxe version with notepad ++ you can change values. You can also create several different copies that are differently named and cycle through them while the emulation is running.