Due to the extension restrictions on the RT it is not yet possible to natively play .flv files in the video player. But the RT ist very well capable of playing .flv as we all know, in form of flash video on websites.
It is possible to edit the whitelist and I just saw that someone even released an app with which you can edit the whitelist quite comfortably.
With all that said: Isn't there a possibility to play .flvs? Maybe an app, that opens them in IE and adds the local address to the whitelist automatically?