Generate FLV "on the fly": duration not set

Hi,
I want to generate FLV files on the fly, as the user request them via web. Whe I get the request I call ffmpeg to generate the desired FLV file with the output to stdout ("-").

The problem I'm experiencing is that ffmpeg does not set the duration metadata to the final FLV file, so the player in the client side does not properly track the video position.

Is there any workaroud for this?
thanks!