I suppose the question to ask is can a passenger be issued with a UFN/PF and be reported through a TIR/MG11?
I don't have any dealings with PF, but with UFNs as long as the notice is paid for I can't see a TOC putting a prosecution case together unless it is a matter separate/unrelated to the revenue issue for example smoking on board etc.
If someone writes in and effectively manages to admit a criminal offence (whilst trying to do something else) if wouldn't surprise me if some TOCs saw that as free money.). I'm not suggesting what the likelyhood of it happenening is, merely that a risk exists. You wouldn't need a TIR/MG11 if there's a useful unrequested confession been sent in.