I found a video that shows Entex Pacman 2 demo mode, and can confirm that it's wrong in MESS. I tried changing a few things like initial RAM contents and register values, but the only thing that changes demo mode behaviour is opcode/interrupt timing.
It means timer interrupts happen 1 or 2 cycles too late/too early, and/or one of the opcodes takes more cycles than stated in Hitachi docs.
Oh cool, glad you found a video. I was going to try to get one recorded this weekend. Do you still need something video'd, or is the YouTube video good enough?