Custom sound changes on playback

I believe you can do this if you customize the instruments.xml file - that's how MuseScore knows which instruments have alternate sounds. In the trunk (future version 2.0), it looks like there are additional facilities for changing instrument within a piece as well.

Currently using MuseScore 1.0, you have two ways to do this. Both are tricky but should work.

Hide empty staff

Note: it will only work if you are ok to have instrument change at system break

1. Create a score with as many staves as instruments needed in your staff
2. Score everything and change staff when you need to change instrument
3. When the score is done, go to style -> Edit General style -> score -> Hide empty staves.

Hacking instruments.xml

File created this way attached.

1. Go your MuseScore installation directory, it depends on your OS and locate templates/instruments.xml or one of its variant in a given language.
2. Open it in a editor.
3. Add an instrument definition in one the group, similar to the trumpet one. Put as many channels as necessary.
   For exemple put the following in one of the group will add an instrument with piano as defaut sound, and ability to switch to picollo and violin sound.
   
<instrument>
<name>My Custom Instrument</name>
<short-name></short-name>
<clef>0</clef>
<aPitchRange>1-127</aPitchRange>
<pPitchRange>1-127</pPitchRange>
<channel>
<program>0</program>
</channel>
<channel name="picollo">
<program value="72"/>
</channel>
<channel name="violin">
<program value="40"/>
</channel>
</instrument>

4. Open MuseScore, and go to Edit -> Preferences -> General -> instrument list and pick the file you just modified.
5. Restart MuseScore and create a new score or add an instrument to an existing one.
6. Use the same procedure highlighted in Change and adjust sounds , Mid-staff change