How would you plan to modify System 7.5 if you don't have the source code anyway? I guess I don't entirely get your question.
"based on System 7.5": Are you proposing to write an entirely new program that "acts" like System 7.5? Or maybe you want to create a software package that uses the System 7.5 system file to boot but replaces the Finder with some other application?
You may be able to require an existing install of System 7.5, then create a sort of patcher that modifies it to be your new package, so you aren't distributing any of Apple's code. There have been plenty of free system mods that work like that and Apple doesn't seem to mind. (Remember ResExcellence?)
"based on System 7.5": Are you proposing to write an entirely new program that "acts" like System 7.5? Or maybe you want to create a software package that uses the System 7.5 system file to boot but replaces the Finder with some other application?
You may be able to require an existing install of System 7.5, then create a sort of patcher that modifies it to be your new package, so you aren't distributing any of Apple's code. There have been plenty of free system mods that work like that and Apple doesn't seem to mind. (Remember ResExcellence?)