It's really not a problem with a shower spray head. It's unlikely that water is going to hit the wall higher than the shower head arm and run down the wall and into the wall. That said...what you can do it get some foam insulation, the stuff I'm talking about might be found in the window insulation section. You're looking for insulation that has a cross-section of maybe 1/4" to 1/2" square, it comes in rolls several feet long and has "peel and stick" on one of the edges. Cut off a piece and stick it on the BACK of the escutcheon plate. Start at roughly the 7 o'clock position, go through 12 o'clock, and stop it at 5 o'clock. Leave the area from 5 o'clock to 7 o'clock open. The idea is that the foam will stand proud of the back of the plate, and when you press the escutcheon plate to the wall, the foam will compress and seal the escutcheon plate to the wall. Any water that hits the wall above the arm and runs down will be directs around the plate and continue running down the wall. Should any water get behind the plate, it can exit through the opening between 5 and 7 o'clock and run down the wall. If you can;t find foam that's thick enough to be proud of the back of the plate, you can apply two thicknesses of foam, one on top of the other. A closed-cell foam would be better than an open cell foam. Here's a quick link to what I'm talking about:

BrianKK: Yes. Some Corain and other solid surfaces are available in 1/4" thickness, but you could use 1/2". After fitting, clean the substrate and the back of the sheet and apply half-dollar size dabs of silicone every 12" or so. Jam some pre-measured props in place (use blocking on the ends to distribute the load) and leave them for a day then tile up to the solid surface.

This is a pic of our shower using clips and silicone. The shower door is open. The silicone is along the wall and the bottom, where the glass meets the Torreon travertine. It is nearly invisible. The clips are drilled directly into the stone tile. There is framing behind it. (edit to say there are wood studs behind the tile) We initially did not have the rubber seal on the glass where it meets the door because the water did not splash out during the "test". The shower head is in the center of the shower but from this photo angle, it looks like it is close to the door. But once a body was in the shower, the water was deflected and came through the crack. Not a ton, but enough to need to wipe up instead of letting sit on the travertine floor. Hope that gives you a visual how clips will hold your glass.

Maybe a bit shallow but I see nothing glaring.