Some network stacks like those in Solaris and HP/UX let you tune the "Nagle limit" in just such a fashion, up to disabling it entirely by setting it to 1. I'm not aware of it being tunable on Linux, though you can manually control the buffering using TCP_CORK. https://baus.net/on-tcp_cork/ has some nice details.

How is what you're describing not just Nagle's algorithm?

If you mean TCP_NODELAY, you should use it with TCP_CORK, which prevents partial frames. TCP_CORK the socket, do your writes to the kernel via send, and then once you have an application level "message" ready to send out — i.e., once you're at the point where you're going to go to sleep and wait for the other end to respond, unset TCP_CORK & then go back to your event loop & sleep. The "uncork" at the end + nodelay sends the final partial frame, if there is one.

There is a socket option, SO_SNDLOWAT. It's not implement Linux according to the manual page. The description in UNIX Network Programming and TCP Illustrated conflict, too. So it's probably not useful.

You can buffer in userspace. Don't do small writes to the socket and no bytes will be sent. Don't do two consecutive small writes and nagle won't kick in.

FreeBSD has accept filters, which let you do something like wait for a complete HTTP header (inaccurate from memory summary.) Not sure about the sending side.