No, I haven't tried that. Good idea, But it rusts. Let us know how it works. What I would try would be 2 stainless steel eye bolts with a nut threaded on the end. Just embed that nut and stem into the concrete/tufa and you can put a picture wire between 2 of them to hang the mirror. You can bend the eye some so it sticks out of the tufa and the stem is parallel to the frame more.

I just bought some brass chain at the hardware store @ $1.00/ft. It's very flexible and strong. I thought I'd slit some drywall tape and put the chain ends through it and cover with crete. I plan on using a 6" piece for each hanger.

I have had success using diamond mesh/lath which is very similar to what you are proposing, Rick. Not only is it good for reinforcing the whole piece, you can thread the wire you plan to use to hang the piece either side of the back, twist it around the mesh to secure it, lay it so that it is half way between the front and back of the piece and then add the rest of the concrete leaving the wire exposed (duh). The wire is then firmly embedded in the piece with nothing showing on the back but the wire. Use a stouter wire, or multiple strands wound together, than you think necessary since, with this method, replacing it should the wire ever break would not be an option. I've had great luck doing it this way. Never a broken wire.

Thanks for the comments.For my heavier pieces,I've been using aircraft cable that's rated for breaking at 1150 lbs!!Yet its quite slim.And not too expensive.I bury the sucker at least 3 inches into the mix,and the free ends are around a piece of 1/2 inch coper pipe.I gues I'm doing "eternity"hangers,eh?
Cheers from here