I'm familiar with the paradigm. I've written some code to talk to dodgy embedded devices that don't "talk" unless they have something to say and I have ended up

I'm familiar with the paradigm. I've written some code to talk to dodgy embedded devices that don't "talk" unless they have something to say and I have ended up having to use timers. It sucks, to be sure. I looked back to RFC1122 to get a feel for the nature of TCP keepalives, and I would agree that they're unsuitable for this application. If we posit that a keepalive mechanism is useful for TLS I'd still argue that a payload is unnecessary because TCP will handle in-order delivery. A NOOP with some random-length padding would have worked just as well.