RTAS will not work as VST. RTAS are ran of a different set of programs. The VST to RTAS converter adds the vst plug to your common files and Pro Tools Libraries so it can be found and acknowleged like a RTAS, but RTAS's are a weird series of programs all the way down to the way they are accessed. You can't make a RTAS to VST converter.
If you like SMACK, Nomad Factory has a Mastering Suite(Analog Mastering Suite I beleive)that I've heard has something equivalent to SMACK in it. But be leary and try a demo 1st. I've been looking for a limiter like the one in Reason, demoed everything, nothing sounds enough like it for what I was trying to do.