Well, I'm not talking about "jailbreaking" (loading one's own software that perhaps may involve defeating treacherous computing measures), but rather wholesale replacement of the control board with a generic open one. That you're wondering if this might be illegal shows just how far manufacturers have succeeded in separating us from the idea of modifying things we own.

Documentation would be fantastic, but it feels like a curation/discovery problem. After a bunch of deep searching, you can usually find a forum where people are discussing various lines of machines at a higher level, what features they have, repairability, etc. Parts websites generally have diagrams and whatnot (no idea where these come from or what it takes to get access to it wholesale). You can usually figure out cross reference part numbers from pouring through eBay/Amazon listings. And sometimes you even stumble onto a wiki or something where someone has documented a specific machine tangential to another project (eg Home Assistant).

Gathering it all up in one place to be useful, especially in a way so people will contribute their findings back so it can be self-sustaining, seems like the hard problem to me.