Originally Posted By byuu

Had a lot of trouble getting all games to detect the SGB hardware when using $6003.d5,d4 to determine 1-player/2-player/4-player mode for masking the joypad ID. Games like Shin Megami Tensei Devil Children were writing out MLT_REQ and then immediately reading the joypad ID. This gives the BIOS no time to react and update $6003.

So I'll keep being evil for now:
  if(packetlock) {
    if(p15 == 1 && p14 == 0) {
      if((joyp_packet[0] >> 3) == 0x11) {
        mmio.mlt_req = joyp_packet[1] & 3;
        if(mmio.mlt_req == 2) mmio.mlt_req = 3;

Pull the MLT_REQ state right out of the packet once it completes. Technically no reason the hardware can't do this, although it'd be quite odd for it to use $6003 at all in that case.

Could it be that it simply halts the cpu in such a case?