I've seen a few running RTT's on a fiberglass shell - especially the smaller and lighter models. But you might see the sides of the shell flex, as well as the bedsides, over the long term. I'd prefer a steel rack myself, but realize that it $$.
I think that "HMR" on Expo has a Maggiolina mounted to his shell - that would be one example you could look into.
1976 FJ40 - green and mostly stock
1996 FZJ80 - not so stock