I've noticed that when using the MinionPro package, some glyphs like ligatures and oldstyle figures would be mapped to another character.

For example, compiling

123 Th fj fi ffj

using pdfLaTeX yields a PDF document that looks correct enter image description here

but if copied, it actually yields other glyphs like Ïth û ö.

I realise that this could be solved using glyphtounicode, but I'm thinking of how this property can be used to actually prevent plagiarism if I make all the glyphs map to wrong characters.

How can I achieve this using either pdfLateX or XeLaTeX? Is it as simple as creating a randomised custom glyphtounicode?