Never got around to trying it myself, but thought it was possible. As stated, the Thundercat forks are longer than the FZS600's, meaning the internals are too.
You could in theory make a hybrid fork from the T'cat lower and FZS upper stanchion. Then cut down the top of the T'cat inners by the required amount and refit. Not sure how similar the positioning of the inner stanchion teflon bushes are compared to one another. I suppose really the fork would be primarily T'cat, just reduced length-wise. But unless you make up several mudguard adapters, you would also in turn have to use the T'cat mudguard, which might(?) in turn space the forks out wider than the Fazer's, meaning T'cat yokes might also be required.... so you could end up with a T'cat front end anyway
