Yes, for all the great things browsers do, it sure is a shame that the only data format they can handle is JSON.

But just because it's a webapp doesn't mean you have to use JSON for everything, it's the other way around, using JSON for everything means that the only place the data model feels natural is the web.

Which is strange because JavaScript has had the Uint8Array that makes manipulating arbitrary byte structures easy for a while, it doesn't feel any less natural than other dynamic languages in that regard. Defining a "signed message with a header" structure in terms of bytes like I described above seems like a no-brainer. If you want the payload data of your protocol to use JSON then that's fine, but that shouldn't dictate the cryptographic layers underneath.