Surely there is an answer to the conspicuosity (is the such a word?) problem - sunlight.
It must be possible to make a honeycomb of modified cube corner reflectors that, when formed in the shape of a fuselage panel, would give a useful 'glint' in most orientations and lighting conditions, perhaps even when thermalling under clouds.
To do this, the reflectors would reflect through angles other than the 90 deg used in cat's eyes. I wonder if this has been tried? It's not the steady intensity of reflection that's needed - just the momentary flash of a single reflector aligning the sun and the observer's eye. The continual manoeuvering of the glider would do the rest.
Just an idea.