Ability to create guide/lead parts (or mixing parts)

I also post this here so maybe we could discuss better.

http://musescore.org/en/node/54386

Example:
I have a score with 4 parts, 90 bars.

What I can do:

• Have the complete score with the 4 parts together.
• Have linked singular parts
• Have a score with part 1 and 2, or with part 2 and 3 etc..

What I can't do (feature request)

• Have a "mixed" part (still linked to the main score), for example:
  • part 1 for the first 12 bars
  • part 1 and 2 from 13 to 20
  • part 1 from 14 to 49
  • part 1, 3 and 4 from 50 to 62
  • etc..