Plain, unglazed, ceramic works very, very, well for frag mounting as well as a lot of other aquatic applications.
Remember, ceramic is basically just very porous glass, and ceramic media is pretty common for filtration.
For those worried about particularly quick attachment, or the unnatural look... try applying a thin layer of silicone or superglue and rolling the frag plug in aragonite sand. Cure the sealant and then mount to the sand.
I'm getting into making plugs for nano-sized frags up to "generous" frags. Part of my process is going to be make them look a tad more natural by coloring the clay to begin with (this, by the way, will be absolutely tank safe. The color will readily match corraline algae, too). And/or rolling with aragonite.
Sadly, my problem isn't getting things to grow on frags, it's getting things to grow onto even a tiny speck of gravel so that I can mount the frags (I have quite a lot of unattached stuff).
