The problem with the reflector, and the thing that didn't occur to me until I had a go at it, is that you view the interference colors by looking at the emulsion side. In the film holder, the light passes through the glass substrate to the emulsion and into blank space where it is ideally not reflected back by the velvet, and interference arises from the RI difference between gelatin & air (2 & 1.5, right?)
So, for a reflector to work in the way you describe, it would have to "disappear" and "re-appear"; switching its position in relation to the emulsion.
Interesting about coherence and use of higher f/stop. That would be an interesting thing to test out, and fairly easy. I don't know if we can tolerate much more of an increase in exposure times though..
Actually, I think the long exposures are one of the most enjoyable things about this kind of photography.
Herbert E. Ives did an investigation on Lippmann photographs while at Cornell (I believe) and utilized a technique pioneered by Cajal. This technique involved swelling the material so that the fringes were brought into the reach of microscopic investigation. He discovered that the fringes only went so deep, but determined that this was due to the sensitizing method, which had been by bathing the plate in dye solutions. So, Ives incorporated the dyes into the emulsion and was apparently able to get much deeper fringes and better color.
He also made 3-color images with Lippmann plates, but that's a tale for another time...