Well, the only comment I can make is that it is supposed to work, according to this comment extract from oggenc/encode.c:
/* libvorbis 1.1 (and current svn) doesn't actually fill this in,
which looks like a bug. It'll then reject it when we call the
SET version below. So, fill it in with the values that libvorbis
would have used to fill in this structure if we were using the
bitrate-oriented setup functions. Unfortunately, some of those
values are dependent on the bitrate, and libvorbis has no way to
get a nominal bitrate from a quality value. Well, except by doing
a full setup... So, we do that.
Also, note that this won't work correctly unless you have a very
recent (2005/03/04 or later) version of libvorbis from svn).
*/
If this continues to be a problem for people, I can only suggest raising it on the vorbis/vorbis-dev mailing lists. The guy responsible for the oggenc code is Michael Smith. He is currently travelling, so any response may be subject to a slight delay.