All boats that are "deficient," meaning less than optimal, have to be "accepted" for racing purposes whenever they show up for a regatta, because they fit within the class specs. You don't have to be a "member" of anything.

If I show up for a regatta with a Hobie 14 with spinnaker, I can sign up to sail in the F16 class if I want to, as a singlehander, as long as my sail area does not exceed the formula class limits; right?

It's not a matter of needing approval from the Formula class people. The approval is implicit in the formula itself.