User blog comment:B214/Question and Answers 6/@comment-24098614-20161024211044/@comment-10330822-20161108104229

Technically, Rias and Issei were able to use the Evil Pieces powers during that fight, just like how Rias was able to promote Issei in the enemy "territory". That's how she was able to use the Rook Piece. Evil Pieces aren't stated to only be functional during Rating Games, Issei just originally needed Rias's permission to promote and needed to be behind enemy lines. It didn't matter whether it was a Rating Game or not. Issei's Illegal Move Trident is basically all-access promotion, Issei can promote without Rias's permission and regardless of whether he is behind enemy lines. Hell, he's even capable of promoting while he is acting as King of his own peerage.

The Rook Piece is usable even if you aren't physically using it for a peerage member. In Chess, Castling works on the Rook "closest" to the King, meaning that technically speaking, Rias could castle on the empty piece because it was "closer".