Hmm. I was curios about this idea so I called British Airlines because I am a member of their Executive Club and I'm flying to the Philippines via British Airlines next month.
According to British Airlines representative they cannot do this because all middle seats will be empty when everyone is to request this option. Also, it is unfair to other flyer's as well.
Once you open that flood gate ... water will start pouring in.
Linkback:
https://tubagbohol.mikeligalig.com/index.php?topic=865.0