does any one knows if such External FPS Limiter exists for FSX working on DX10 that would also work well on Windows 7 x64 w/SP1?

Looks like there isn't one for your combination. But there are rumors, Nvidia will also have FSP limiter included in next version of their drivers and I somehow expect drivers for all operating systems and DX versions will have this functionality included. So nothing else left but wait and see.