I'm very late to this question, but I think I have something to add:
PGN was not designed for games that are played according to FIDE rules, which do allow
illegal moves in game scores under certain conditions. PGN was designed for games that are worth recording.
(Read that again. It may sound insulting -- but it's actually in the specification of PGN)
The PGN specification, section 8.2 (which describes movetext section: the section of
the PGN files where moves are placed) says:
Because illegal moves are not real chess moves, they are not permitted
in PGN movetext. They may appear in commentary, however. One would
hope that illegal moves are relatively rare in games worthy of
While you are able to add illegal moves to a PGN file, you can't be certain
that any PGN-reading tool will accept the resulting files, and display it any more.
There used to be a chess database program (on Windows) that allowed illegal move
entry. It was called CDB, and written by Peter Klausler. I used it more then a dozen years ago, but I've lost track of it -- so I don't know if Google-hits I get now are for the same program, or if it even works on modern Windows releases.