@teller In my advmess.rc file, I have:
input_map[key_lshift] keyboard[0,lshift] or joystick_button[0079_0006_3,2]
input_map[key_rshift] keyboard[0,rshift]
input_map[key_shift] auto
bbcbp128[home_pi_retropie_roms_bbcmicro_thrust]/input_map[key_lshift] joystick_button[0079_0006_3,2] or keyboard[0,lshift]
bbcbp128[home_pi_retropie_roms_bbcmicro_thrust]/input_map[key_rshift] joystick_button[0079_0006_3,2] or keyboard[0,lshift]
Obviously, the joystick buttons will be different for you. There's unfortunately not a lot of consistency overall between everyone's setups, so experimentation is probably the only way to get there.