DISCLAIMER: I haven't actually looked at the MAME code.
I have no idea about 1.), but I guess that parts of 2.) should be in the realm of possibilities? Some games/machines in MAME already support 'highscore saving' which specifically saves the highscore to your disk for later retrieval on the next startup, or 'save states' which attempts to save the entire state of the machine (including but not limited to highscores) to disk for you. Perhaps by looking at the pieces of code that do these things, you can get an idea of how to go about doing the things you want with the highscore?
If this is not what you are looking for exactly , then the The Internet Wayback Machine Archive has an archive of a (now dead) website called "Let's Hack Arcade Games" that has quite a few tutorials on how to go about using the MAME debugger/disassembler to reverse engineer a machine/game, with a focus on the Z80 processor and Donkey Kong as an example, which might be useful in finding the stuff you are looking for in the games/machines you want.
https://web.archive.org/web/20161106154 ... .com/?p=18