I too was on this quest a while back, and all I found was the :
RG521, rare as you said.
RG470FX, seen one once, that was about it. before I was into Ibanez.
USRG10, I think thats the model. another rare one. US made, not exactly cheap. on EBAY from time to time...
Thats about the only RG hardtails I could find.
Ever since I looked into it, it seems like LOTS of people have done the same thing. Maybe Ibanez will listen.
Ive thought about blocking the trem on one of my 550's, but that kinda defeats the purpose, you still have that hunk of metal there(not that I hate trems, just a change would be nice), and you lose the string-though-body sound(more sustain, etc...)
You can find all these on EBAY, but it depends on what your willing to spend...