I read the thread in more detail. Since the bases of the reflector and the flare mounts can be disassembled, I think all you would need to do is to build a product similar to the Trek Madone Light Mount Varia Insert, except make the insert a GoPro attachment instead. Does that make more sense? If not, I can take a picture of what I mean tomorrow.

Right now, Trek makes an attachment for a front GoPro mount on the 2016 Madone, but there is no way to put a GoPro on the rear with the current bases provided which is a shame because that would be such a great and convenient spot for a GoPro (even better than under the saddle using current mounts like the K-Edge).