> The problem is that formats like JSON are designed to be human readable and writable. Length prefixing is a non starter here.

Canonical S-expression are both human-readable & length-prefixed. They do this by have an advanced representation which is human-friendly:

    (data (looks "like this" |YWluJ3QgaXQgY29vbD8=|))

And a canonical representation which is length-prefixed:

    (4:data(5:looks9:like this14:ain't it cool?))