Sure, you can do that. Userspace code can use any ABI it wants, or none. But again, why, what do you gain?
And regardless of whether it's "the" ABI or merely "a" ABI, that ABI presumably has a representation for strings and allows passing them around - and while you certainly could use a different representation in your program (or in the OS internals) and transform strings back and forth when calling the OS (or when receiving calls from userspace), you probably don't want to. At which point we're back at needing a way to write strings in an in-memory format to OS-standard files in the filesystem.