Why JSON? Just use a simplified subset of HTML and ignore styling+scripts for commandline. And for GUI/Web you can still offer the full HTML+CSS+Scripting-experience.
I don't see how yet another ui-definition-format will help to make things simpler. And what UI does a manpage need? Manpages are documents, they have no real ui.
Anyway, this idea is free, so feel free to take it and do whatever, even full HTML (: