Well, I thought about going this route and decided against it.
Personally, I would use a carriage bolt through a set of square bars for this application, or add clamp-on Thule saddles (which I did).
The thin, slide-in T-bolts are made for lighter equipment mounts, like bikes or snowboards. The longer you make these, the less you will be able to trust them.
I get the impression that Rhino Rack's channel system is much more robust and maybe up to the job. Our Aussie friends may be able to comment on this.