Well, you'd change the ship's destination and step into a warp that isn't there yet. The .j2l would have about as many ships in it as there are planets unlocked, and you'd go through the planet select screen back to one of the ships, and each one would have a different end level event.
Though I had a proper look at your example, and you could probably 'reset' the once-only secret level event by reloading the hub every time you finish a secret level. Just put an end level event next to the secret level event. Mind you, I could probably just use triggers and squish it down from 10 or so hub levels to one. That would make things a bit more complicated, but I think it would work better and save space.

EDIT:
Also, beating a boss in a secret level sends you to the next level instead of back to the hub, so that's another reason to have more than one.