Hi there! I've only used Rosegarden a little bit and quite a while ago, but I'm a musician so I know what you're asking. I don't have Rosegarden in front of me at the moment, so improvise if you need to.
According to the documentation that I can find, it looks like what you need to do is actually transpose the notes to their written pitches (instead of concert) and then set the "Transpose" property of the segments to compensate.
Transposing the notes:
http://www.rosegardenmusic.com/doc/e...html#id2559156
Setting the Transpose property:
- Segments:
http://www.rosegardenmusic.com/doc/e...ameterbox.html
- Tracks:
http://www.rosegardenmusic.com/doc/e...arameters.html (this might just set the default for new segments though...)
For your Bb Cornet example, you would transpose the notes up a major second, then set the Transpose property on the segment to a major second down (i.e. -2 semitones if it asks for it that way). You'd have to do that for each instrument's staff individually, I think. So for Eb Horns, you'd transpose the notes up a major sixth and set the segment's Transpose to -9; if you want F Horn parts instead, a major fifth, and -7. It's probably easier for composers who write at written pitch instead of concert. *shrug*
Hopefully this works! There might be a better way, but without the program at my fingertips to experiment, I couldn't say...