in effect that the K can be captured en‐passant on its starting square
This is exactly how I implemented castling in Fairy-Max and the Interactive Diagram. So these would not castle away from an adjacent enemy Imitator, even if it just had landed there through a Knight move. This is an ambiguity in the definition of castling, however. The condition for legality of castling could have been formulated as "when the King could be moved to any square between and including its origin and destination without exposing it to capture". As this describes a stay at the origin as a (null-)move of the King, a Joker should use a King move to capture it even there.
This is exactly how I implemented castling in Fairy-Max and the Interactive Diagram. So these would not castle away from an adjacent enemy Imitator, even if it just had landed there through a Knight move. This is an ambiguity in the definition of castling, however. The condition for legality of castling could have been formulated as "when the King could be moved to any square between and including its origin and destination without exposing it to capture". As this describes a stay at the origin as a (null-)move of the King, a Joker should use a King move to capture it even there.