Not exactly sure what benefit it would be to them, the 757s only have 177 seats in comparison to the A320's 174, and they'd burn a considerable bit more fuel. Perhaps if an A320 went tech they could slide in and do a rotation if there was the time remaining, but with such a low density layout, I can't see them using them in inter-European routes when they have the A320 and A321.
I'd understand if the 757s were high density with maybe 200-210 pax, but not 177.