A lot of waxes/sealants still contain some silicone, as it is used as the "filler" which hides all your minor scratches and possibly some swirls until the layer wears/evaporates away. I figure NXT must have "some" silicone in it...as it tends to hide the smallest of scratches on a car's surface.
In terms of not using it in a clearbra...I can't see why not. I mean, one could argue that the clearbra is designed to breathe and silicone would "block" that ability...but paint is supposed to breathe as well. Beyond that, any silicone content is temporary, so it would last just as long on the bra as the paint. I mean, thus far I have used a cleaner wax (TurtleWax re-branded as Zymol), a "polymer" wax (Meg's NXT), and a proper sealant (Wolfgang) on my clearbra and it looks fine to me (if not better "waxed" than "not waxed").
Really, the only thing I'd never do is use an actual automotive polish on a clearbra...no idea what would happen there if you tried, but I doubt it would be good at all (considering polishes take off a minute amount of paint, and would probably just marr the crap out of the clearbra).