If they left contact info or site info on the FAQ/readme/release notes that might be an idea.
A PM out of the blue can work but might not do so well.
If it is a technical question then a general thread, maybe with the author's name in the post could work. If nothing else many people might have contributed an idea or two to the hack, read the post hack analysis, done their own post hack analysis or otherwise worked on the game in question. Answering the general theory on how something is done
Language questions work much the same as hacking ones though such things can be a bit more subjective if someone else answers, this is common to all language discussion though.
Questions on how a hack might have changed the game from a game design perspective are rare but not unheard of. These probably will warrant the original author to respond unless they otherwise discussed something in a post hack analysis, readme or in general.