Arranging for Flex Pieces

I think the OP assumes everyone knows what a "flex piece" is. I think it means this:

First write a set of generic parts (five in his example) without assigning them to specific instruments. They are just parts that would work fine together. Call them part1, part2, part3, part4, part5

Then, assign instrument parts to these generic parts. For example:

• a trumpet part that plays part1 (i.e. notated in treble clef, transposed one step up for Bb instruments)
• an alto sax part that plays part1 (i.e. notated in treble clef, transposed a sixth up for Eb instruments)
• a trombon part that plays part3 (i.e. notated in bass clef, not transposed)
• a tenor sax part that plays part3 (i.e. notated in treble clef, transposed an octave and one step up, for Bb instruments)
  etc.

And I assume the point is that when you make adjustments to part3, the corresponding Trombone and Tenor sax parts should automatically adjust. (possibly, it should not be possible to make changes to the instrument parts, only the generic part, or perhaps it should be possible).

FlexParts would mean a new type of "staff linking", similar to, but not the same as the staff linking for TAB staves.

Obviously, there can be more advanced features also, like
- Tuba plays part4 one octave lower
- Baritone Sax plays part1 one octave lower, but overrides some notes of part1 with others.

This feature does not exist in MuseScore, as far as I understand. But it would be very useful, in my opinion.