I've just returned from the consultation meeting at Barton and put to the technical chap there (sorry, forgot the name) a proposal very similar to Beagle's. He agreed it would be an effective interim solution if you assume that existing Mode C transponders would all be replaced gradually so extending the 1090MHz saturation deadline past the date at which Mode C would have disappeared (no timescale given though). Interestingly, he said there had been the same solution proposed at Lasham.