The closest you'd ever get to that is the late, lamented Asian Fantasy Film Expo (2000-2002), held in New Jersey.
What about Power Morphicon?
That was really a Power Ranger-centric event... not really about toku in general, and while I wasn't there, I'll assume Super Sentai wasn't very prevalent in that event either.
You assume wrong. Everyone above 15 or 16 there was a toku fan or posts on Rangerboard and had knowledge of Sentai. Although there were no Sentai focused panels, S
While I'm sure something like this could take off, it's sort of a pipe dream unless someone really has the pull to put it together.
How about California?