I took a similar (but much simpler) approach to the same problem.

I have a clip-on taillight I keep in the pocket of my bag. the problem is there is nothing on the back of my bike to clip it to, and I didn't want to put one of those ugly clip-mounts on the seatpost.

So I put a zip tie spanning the saddle rails. Now I can easily clip the light under the saddle when I need it. The light is muuch more conspicuous when it's on, and the mount is damn near invisible.