[fulafel]

MongoDB uses this to express decimals as JSON: { "$numberDecimal": "<number>" }

[e12e]

I suspect creating a specification based on what mongodb does/did might be the better approach - but a quick search for "typed json" turned up:

https://www.tjson.org/

Not sure if I'm a fan of the syntax - but some kind of open, sane, standard would be nice.