NICE pic!
Yeah... it will plate out horizontal asap... thats where the light is. But you can certainly mount it any old way you want at first :)
Kinda did this a few times... they will end up as big horizontal spiral plates eventually..
I like the idea presented though, purposely mounting a small frag vertically will get you growth out of it in an interesting pattern to start with.
The longer I have these and a variety of them in my tank the more I appreciate the surface of a Monti as an amazingly beautiful thing. The undulations and polyps are like looking at a strange planet that glows....