Have a option like the "disable job" option to mark a phase/job as
e.g.

• non-blocking
• asynchron
• non-waiting

This phase/job is executed in normal manner of the pipeline but does not block the execution cycle of the surrounding multijob.

For example:
A multijob M with three phases:

Phase 1: A
Phase 2: B
Phase 3: C + D

M has several post-builds (with conditions). B,C,D are multijobs itself.
C is a multijob containing jobs (e.g. maintenance jobs), which can take very long. This job can be executed asynchron, so that the surrounding multijob M can finish an execute it's post builds and a result can be generated.
The result of C is not important for the result of M, but should be shown within the pipeline of M.