It kind of depends on what coral you wind up using. If whatever frags you want to attach have some kind of stone base, then you can probably superglue them to the back and let them spread from there. Same with star polyps or encrusting gorgonian; just put the glue on directly on the bottom side of their mat. The only catch is that this needs to be done on a fairly dry surface - aka not under water.
If you wind up using Xenia or some coral that attaches very quickly, Anthony Calfo suggests in his Propogation book that is crafty and useful. Take a length of PVC with one end cut at an angle to roughly match your back wall, and long enough to reach where you want to coral to go, and to extend Out of the tank. get the pipe propped up right where you want it to go on the back, with the top end of the pipe leaning against the tank rim, and drop the Xenia down the tube. It might take a bit of creative thinking to prop it up just right... The xenia should settle against the glass; its doubtful that the angle cut will be perfect, so there will be small gaps to allow waterflow around the xenia, which will keep it healthy long enough to attach. There is a slight risk that it will attach to the pvc rather than the wall, but there arent a whole lot of other options for soft-bodied, unattached corals.
The other option for something like xenia would be to put a colony that is on a rock up against the back wall, with the xenia touching the wall, and just wait for it to grow up towards the light and spread up and out on its own.
