I flew from IAD to LGW this past summer, and while in London discovered FT. I called up for a retro plat challenge, and both of my outbound legs had already posted. The CSR made no mention of not being able to make it retro, and on the 90th day of my challenge, I crossed the threshold. I had to cancel an earlier flight and almost didn't make it, but my first legs did credit retroactively.