XSPF: Can I specify audio or subtitle tracks?

[StarManta]

I have a need to generate XSPF playlists with start and stop times, and all that. It appears that the VLC extension of XSPF supports a number of different options. One that would be particularly useful would be selecting a particular audio/video/subtitle track. Does anyone know if this is supported and, if so, what the exact syntax would be? Would it use the track's name or an index number?