I've tried setting the value for bgfx_shadow_mask but I'm not having any luck with this value being picked up by MAME when launched.
bgfx_path bgfx bgfx_backend auto bgfx_debug 0 bgfx_screen_chains crt-geom-deluxe bgfx_shadow_mask slot_2_4x4_rgb.png bgfx_lut bgfx_avi_name auto
The value for bgfx_screen_chains is being read and crt-geom-deluxe is being applied to all games by default. However, the shadow mask appears to default to aperture_1_2_bgr which hasn't been set anywhere.
The slot_2_4x4_rbg.png file is located in the artwork\bgfx\chains\crt-geom folder. I have also copied it to the artwork folder but still no luck.