Does anyone here know of a version control system that keeps its data in the tail of files?

I don't know what you mean by "tail of files", but it sounds like you're asking for a system that keeps the version data in the file itself?

That would be a very specialised system. The RCS would have to be familiar with the format of the file, and it could only work with file formats that have a concept of comments. It wouldn't work with a basic text file, for example.

I doubt such a system exists. It's just too complicated and limited.