Co-resident matched sports med last year. Here's the hearsay:
It is likely the most competitive fellowship for pediatrics, though this is due mostly to its small size. (And the fact that most peds fellowships are not at all competitive).
Some (but not all) FP Sports Med fellowships will accept peds applications, however they greatly prefer FP residents. For example, I think our resident interviewed at 15/16 of the peds programs he applied to and 1/5 of the FP programs he applied to. (Those numbers are made up, but reflect the general trend).
However there's good news: Sports med is a small world. Publishing case studies and going to conferences and shaking hands and making friends seems to go a very long way.
More good news: A lot of research is being done on injury prevention now that several injury databases (e.g. the NEISS) started in the last 30-40 years are readily accessible. These are projects applicable to sports medicine where most academic centers probably have someone around who is doing that kind of research, even if they don't have a sports medicine department.