Today I made some calculations and gave it a thought, and came to the conclusion that a WC with Ryukyu is impossible.
Have a look at my Ryukyu HTTT AAR: http://forum.paradoxplaza.com/forum/...her-Ryukyu-AAR
There I got incredibly lucky with the start, horribly exploited game mechanics and even with that, still had a hard time surviving (I ended it when school became too troublesome). I had an army of, what, 3000-5000 men in the end? Most wars will have to be fought by your vassals, since you can't field a proper army and have no cores (thus getting rebels which you can't fight, so your allies will have to do the job).
That said, the proper game (after the surviving period), starts when you get your first core province somewhere else. That's already -50 years from the time you have for a WC. Ryukyu's sliders are also horrible IIRC, so getting ready for westernising will take a lot of time. And in addition, the Europeans take hundreds of years to arrive to the Far East, so you have to fight your way to them.
There is only really one way to make a WC: Vassalise everyone and abuse the current "always accepts annexation" bug. You'll face rebellions, but you'll expand. If you're mega-lucky, you'll make a WC before you collapse. I bet that not even Prawnstar, the one who nearly conquered the world with natives in IN, can't do it.
I end my report with the conclusion that making a WC is an impossibility with Ryukyu. I have a small hope that someone finds a hole in my report that allows a WC, but it's not looking good.