Modifying SOAP Headers, providing additional Information

I was reading the Soap Headers thread by Markus Piff and I was wondering
if this is useful for our problem, too.
What we want to do:
If a BPEL script calls a service, ODE should provide additional
information to the JAVA engine/service, e.g. the execution of a plug-in
or the return of a specific result.
How do we achieve this? Is the SOAP Header modification a valid idea or
are there other, more easy ways?
Additionally, we wonder, how ODE manages service calls internally. E.g.
there are 5 running instances that request services and results should
be returned to the caller, how does ODE map these information?
If you need additional information, please feel free to ask!
In advance, thank you for your help,
Benedikt