It could be implemented on any train, even HSTs (just have to be able to select which doors to release), but it would cost money and need a lot of work (probably a near total replacement of the CDL or door control circuitry). It's not been retrofitted to any trains, but that's because it's never been needed (since they have "grandfarther rights" to stop at stations that are too short) not because it can't be done. There are other issues aswell though. Especially for the Mk4s, stock availablity is an issues, I doubt enough Mk4s are spair to extend existing sets even by one coach. Also, adding extra trailers reduces performance, and this can make capacity issues worse. The 91s, arn't special when it comes to performance anyway (they lack tractive effort and raw horsepower for starting, due to low weight, and actually are slower off the mark then HSTs, and way behind modern units like 22Xs). You've also got problems of long trains fowling junctions etc, although since regional Eurostars ran on the ECML that can't be too much of a problem, not to Leeds anyway.