Needed Info When Exporting to MusicXML
1) I believe Transposing Instruments, when exported to XML, should have a "transposing" element (within the "attributes" of the first measure of a given part) that contains both "diatonic" and "chromatic" steps, so that an importing facility can deduce the correct pitches, mainly for MIDI playback. I'm using a 0.9.6 pre-release, and couldn't find this info in the exported XML.
2) Although there are tempo strings, sometimes one needs to put a metronome mark instead. It's true, the tempo value is available on selecting the tempo string, but a mere metronome marking might be a nice thing to have. MusicXML does cater for one, too, as a separate element, apart from the "sound" element.
3) A final request: Instrument names containing the Flat character can cause problems when imported via XML, since the same font might not be available then. So, I suggest converting that character, during the exporting process, to a simple "b", e.g. "Trumpet in Bb", and let the importing facility convert that as needed.
Hosam Adeeb Nashed