KotDB: In Shogi the promoted pieces revert back to their unpromoted shelves when captured. While promoted they're marked with a circle around the unpromoted symbol. This is if you use the Roman letter marked pieces as I do to play. If you use the Kanji marked pieces they're red colored when promoted and the promoted piece has a different name.
Either method would seem to work for this variant you're talking about, though just using a red dot on the promoted piece would seem like a real easy thing to do. Especially if the captured piece is going to revert back to a Pawn and the player might have a choice of captures to make.