First, please don't cross-post. I'd go delete the other post.
This is indeed a straightforward project. With a Mega there are enough pins to drive the displays directly, they wouldn't even need to be multiplexed. I'd think most optical interrupters could be treated as a digital input signal as opposed to analog.
So yeah, simple, but we all have to walk before we run. What have you tried so far? Check around for tutorials, GIYF.