If one were adding this to the kernel, maybe rather than stipulating a specific protocol, assign them magic numbers; maybe JSON-RPC could be protocol 0x4a534f4e. An app could ask the kernel “which protocols does the pty master support?”, and get back a list of integers. And then it could ask to open a sidechannel for protocol 0x4a534f4e. So the kernel doesn’t even have to get in the business of mandating a specific protocol (whether JSON-RPC or CBOR or 0MQ or whatever), it just opens side channels with a meaningless (to the kernel) integer to identify its protocol.
Passing the process ID and user ID might be helpful to improve security of the terminal emulator, too. If the sidechannel is a UNIX socket then it will do this (with SCM_CREDENTIALS), as well as pass file descriptors (with SCM_RIGHTS).
