you could probably use reef safe mortar mix if you would never need to re-position the coral, but you would have to let it set up in air a bit (small ball) and have to hold the coral in place for a quite a while until it holds. marco rocks sell some serious stuff. when it's cured it's much stronger than even your rock. Marco states theirs cures fast under water, although i haven't tried it in that application. Only dry rock application for me.
Hmm... I wonder if I could use a two fold approach. A small amount of marcos making direct contact with the rock and frag, but also a larger amount of the Tunze epoxy on the outside to temporarily hold it in place while it cures and the smooth off the outer rough edges.