Many thanks for the WebM support in mkvmerge
Although, not your problem, Vorbis in WebM cannot be played by the WebM DirectShow filters unless I add "--timecode-scale 1000000 --disable-lacing". Plays fine in VLC though.
Probably a limitation of the MKV parser code in the WebM filter, might peek thru the source and try and come up with a patch.
Cheers,
-Nic
EDIT: Oh, although probably of no use to anyone as there's other solutons:
http://nic.dnsalias.com/ivfenc.zip is ivfenc with AviSynth input support (do a ConvertToYV12() at the end of the script)