Doh. Commas are supposed to work, but I broke that at some point.
Fixing for next build.
FYI it's of the form <Command>TrackID,TrackID,TrackID where track ID can be a number (1), a range (1-10), an exact name (Guitar), or a filter (*synth*). E.g. S*synth*,*atlantis*,*moog*,*roland*.