Bernhard...
I use the old style external flash with a slanted slide as a mirror. Well it's better than nothing.
When I switched to the stand I currently use, my old "flash" method could not be used. So while thinking things over, (and having a friend machine some needed parts), I set up an arrangement as you have done. For about 8 months I used it, and it worked superbly. The only real annoying aspect was that the flash unit was a bit "in the way".. I would often inadvertently hit it with my hand. Alignment was important, but not hard to accomplish. In my setup it was necessary to add a small amount of diffusion to the front of the flash as in some cases a light/dark "pattern" of the fresnel was seen in the image. I used a larger piece of glass (actually a pellicle reflector) so that I could leave it in place and use the tungsten lighting when desired (and not have uneven light from the edge of the slide in the light path).
All in all, it is a very effective way of implementing electronic flash. With a little additional refinement and repositioning of the flash it could have made a fine permanent arrangement my purposes.