Whereas lisp-data files usually contain just one Lisp form.
That's a bad assumption. It just isn't true. And I don't see any difference to elisp here, what did you have in mind?
So e.g. any
"defun navigation" would jump between the direct children of that form.
There's no inherent structure to lisp data in general that supports any kind of navigation, except maybe basic sexp navigation. There is nothing we can parallelize to a defun. But if you know same subclass of lisp data that does have some of those properties, we can a special major mode for it. Doesn't have to be anything special, mind you, a couple of lines will probably do what you might have in mind.
João