I am afraid it is not yet supported, as far as I could find multi instance characteristics supports only input data.
I gave it a try and made some modification and looks it could be done rather fast, of course I focused only on one part of it (sub process only) but it worked as expected. So creating a jira for this is good idea I think.
Thanks for the information. I'll log that in jira when I can. Meanwhile, I workaround it by adding the changed items back into the collection and removing the existing.