I wouldn't be to concerned about any copyright claims, if there was a legal statute for modifying files on your own computer Bethesda would have used it during the payed mod debacle and I'm fairly sure they have a more prolific legal team.
If they did want to make a claim based on modification of code I'm pretty sure they would have to step up the ladder and have it go through RPGmakers Developers(ASCII, Enterbrain, Agetec, Degica) as they are just licensees and clearly aren't making much original code with the laundry list of add-ons the game uses. Cross referencing all of which probably being the majority of the bugs you encountered I'm sure in your stint at bug testing you mentioned.
If they did want to make a claim based on modification of code I'm pretty sure they would have to step up the ladder and have it go through RPGmakers Developers(ASCII, Enterbrain, Agetec, Degica) as they are just licensees and clearly aren't making much original code with the laundry list of add-ons the game uses. Cross referencing all of which probably being the majority of the bugs you encountered I'm sure in your stint at bug testing you mentioned.