I have seen the reference to Korchnoi asking whether he could castle but never an explanation.

According to this source it was a reasonable action.

"Korchnoi confirmed he did ask the question at that point, explaining that the Russian chess rules left the situation a little ambiguous, and it was the first time the situation had occurred in his games. Considering the levels of tension surrounding the match and this game in particular, Korchnoi thought it best to confirm with the match referee before making the move."

Is the explanation plausible? Does anyone have knowledge of different castling rules in Russia at the time?

Perhaps someone has a game where castling would have been best but it was disallowed?

It seems that this unlikely set of events did occur, however, I would not be surprised if this was a psychological ploy used by Korchnoi to unsettle his opponent. There always was a lot of gamesmanship employed during Karpov-Korchnoi matches and his explanation seems highly implausible. I can't believe that a long time contender for the crown did not really know the rules surrounding castling. These situations with the rook passing through an attacked square or starting on an attacked square arise often enough in a playing career. Just my take on it, the only people who really know for certain are Korchnoi and his seconds for that match.

p.s. Thanks for the link, great article.