Ok maybe someone can look at this with fresh eyes. I've found a byte for BEL with data, but I can't figure out the pattern:
At emulator_multicpu.exe+174CF0 there are the following data cycles:
-------------------------------------
both "press start"/coins inserted in attract mode >> 255>247
both started >> stays at 223
p2 started only (coins remaining) >> 255>223
p1 started only (coins remaining)>>255>247>239
p2started no extra coins>>239>223
p1started no extra coins>>223>239
-----------------------------------
The main thing is I can't find a video of the game's attract mode, so I don't know how the cabinet behaves. I'm guessing from this that the start buttons only blink when coins are inserted. I can also assume that when there are credits remaining, both start buttons blink during the attract mode as you can start the game with either button, even if it's single player. The question is, does it do like traditional sega games where the start button remains unlit after you press start? The bitmasking is also weird. Usually start 1 is 4 and start 2 is 8, so you'll see 12, not 8 subtracted and added when both lamps blink. 223, which I think is both lamps off, subtracts 32, but in attract mode, it alternates between 255 and 247... wtf?