This is an interesting post.
Polling Raspberry Pi GPIO is very slow,< 1mhz and funnily enough RPi B+ can poll the GPIO pins much faster than RPi 2 or 3. However your idea is very interesting, I have had moments of madness where I thought it would be fun to interface directly with the cpu for various reasons. But it always seemed liked more work than I was prepared to do. I however have a suggestion for you that might work...
1. You don't have to "find" the high scores etc. in the cpu, because the machine tells you all you need to know on the display!
2. Intercept the display data.
3. identify the data you want, like the high scores.
4 send the data to your pc/rpi or whatever.

The previous post, the ram idea is also interesting, but more wires, might be cool to play with if you had an extra wpc cpu to play with, maybe some soldering ... Also probably not needed to clone the ram, but see how long you can delay comms between the ram and the cpu before it complains, to give you time to read your addresses ..
Good luck

Another board will be better, if you need to read the pins faster than 1Mhz. But also you will need to wait for the clock signal and then read the pins, so half that maybe 1/2Mhz.
Your idea is cool , it got me thinking ... but why interface with the ram directly, it is soldered onto the board, but the CPU chip itself is socketed ,so easier to interface with, wait for it to write to the ram address and listen in.

Yes but you have al the time in the world (the display refreshes at 120hz) all you need to get high scores is the font and location on the screen used to display the scores and you would be looking for "1)" or "2)" etc. in all the frames it pushes out. It will be different for every machine. It won't be easy, but it won't be super hard either.

Not to sound overly positive but if you can read it, you can program a machine to read it.

Yes I agree a single universal solution would be the best. Nice ideas all round. Might be a fun project. I might try.