Would it make sense if a flutterdash (disguise) tag was made for Fluttershy’s disguise as Rainbow Dash, or is it already made? So quick update, there only a handful of her in the disguise. Nevermind.
But the ship tag should imply disguise and the episode Magic duel.
@TheGamerPainter axillism (NSFW) covers the “arm over” part here well enough. If you happen to find cases of same, but with the elbow, put them in a gallery and we can think of a name for that special case.