Hm... I see that MC6847's color artifacting is enabled via a UI config option, which is available to the code as a port:
INPUT_PORTS_START( m6847_artifacting )
PORT_CONFNAME( 0x03, 0x01, "Artifacting" ) PORT_CHANGED(artifacting_changed, NULL)
PORT_CONFSETTING( 0x00, DEF_STR( Off ) )
PORT_CONFSETTING( 0x01, DEF_STR( Standard ) )
PORT_CONFSETTING( 0x02, DEF_STR( Reverse ) )
/* are we artifacting? */
artifacting = input_port_read_safe(machine, "artifacting", 0x00) & 0x03;
if (artifacting == 0x00)
...else (artifacting != 0x00
), proceed to produce the artifacting effect.
I could do something similar, add a UI config option so that joystick is only mixed to the keyboard if the user chooses that.
(Unless, of course, there is already something like that already implemented in MESS' core.)