I suspect that the PGN standard does answer your question, though perhaps indirectly.
220.127.116.11 says that the content of the Result tag "is always exactly the
same as the game termination marker that concludes the associated movetext."
8.2.6 says: "The game termination marker is a symbol that is one of the
following four values: "1-0" (White wins), "0-1" (Black wins), "1/2-1/2"
(drawn game), and "*" (game in progress, result unknown, or game abandoned)."
So, no +- or other similar combination allowed. "1-0" is allowed, but seems fundamentally wrong for a bye game -- unless you keep the score to yourself and only use it with PGN software that doesn't interpret that as a real win.
It seems safer to use "*", and add a non-standard PGN tag to indicate
that the game was a bye game for a particular player. (That is basically
what ferdy suggests in his comment, but it seems unwise to extend an
already established and defined tag (i.e. Termination) unilaterally.) This seems to be less disruptive of already existing software.
If the information is for human use only, use "*" and add a movetext comment to the effect that this is a bye game.
PGN was not designed to support FIDE rules. It was designed to be
extensible, at least as far as tag pairs go.