I tried that at the time, but all of the tools I tried either crashed or only recognised the audio track (tried ffmpeg, QuickTime in OS X and Windows, Handbrake, VirtualDub, MKVToolnix, and mpegstreamclip plus a few others; mostly ffmpeg-based stuff crashed AFAICT - most of the others either errored out or just listed the audio track).

I ended up recovering most of the footage by re-recording the screen whilst it was playing in Windows Media Player. Not ideal, but it was a low-ish quality capture of a webstream anyway, so any quality loss wasn't particularly important.