The barbus that sends all the messages to the reel and lamp boards is to put it mildly f*****g fast so would need a high speed data capture system to even start to sniff as to what its doing.
Or a second MPU5 board hooked up to the barbus. I wrote a small monitor program on the second board that just relayed everything on the barbus to the dataport which was being logged by the PC. This worked effectively because the data transfer on the bus is pretty bursty giving time for the slower relay to catch up.
A little tech info to save you some time. If you use the Barcrest chip select configuration (look in an EPROM). The 68681 (dataport) sits at 0xffffffe0. The barbus uses a 68340 on-board serial port. Set it to run at divide by sixteen of system clock (CSRA=0xEE) - 8 data bits and no parity. There is a press-to-talk transistor for the barbus connected to the RTS pins of the 68340. You won't need this for simple monitoring but obviously you should make sure you program the pins to be general purpose IO and disable CTS/RTS functionality.
So what're you waiting for?