Hmm, okay. After byteswapping and reflashing P1, I am getting the system booting into a garbled display that seems to cycle between three/four different (garbled) screens, with visible transition effects going on..... I'm not quite certain what DDP's attract mode looks like but this sounds like a step in the right direction at the very least. Before byteswapping I was reliably getting the PGM to boot into a blue service menu (it seems like "blue menu" means "cart detected" where "red menu" is "no cart") so I think I might be on the right track? Going to redo the tile EEPROMs when I have the time and see if that gets me anywhere.