This spec seems to supersede OggMNG (which is good, since this spec seems much more likely to be implemented), but isn't written with one of OggMNG's use cases in mind, namely graphical subtitles and other kinds of overlays on another video stream. To support this case, you at least need the ability to specify a transparent background colour, which seems trivial enough. Some means of specifying whether an OggSpots stream is intended to be an overlay or not might also be nice, and this was discussed briefly in the OggMNG spec, but no decision was reached.
Anyhoo, I think that this is an important feature for this kind of codec, so I just wanted to chime in and make sure it wasn't forgotten. Subtitle support in Thoggen for instance, is blocking on this capability.