Like you I don't see how a change in protocol is requried, an update to the RFC to say something SHOULD time out the connection if the send window is zero. That said I haven't read the specs with a toothcomb and perhaps there's something about how you MUST NOT drop the connection if you're getting keepalives?

Get Cisco and Juniper to implement it and that's 75% of LINX covered at least, I assume other exchanges have similar equipment makeup.

It seems reasonable behaviour to me.

It doesn't prevent the problem of the malicious BGP peer of course, but we know that already - if they choose to ignore your messages (while being happy with a high send-window) but continue to send keepalives you're equally screwed.