Background Process

Situation:
I'm using the normal LibreOffice installment (background) as a document converter and "print spooler" using -headles -p -pt and so on.

It is called from "my application", which uses a document queue. The normal executable can be run as blocking or non-blocking in the background. For my use it needs to be blocking, each document in the queue is to be handled fifo. This is required for my application.
The drawback is that i also use LibreOffice for my day to day business.

Question:
So i installed LibreOfficePortable for this task. But:
LibreOfficePortable.exe's seem to be non-blocking only. So the queued documents aren't handled fifo (if handled at all) and LibreOfficePortable may crash or its results get unpredictable.
I also tried to run the swriter executable directly, the results were unpredictable too (some documents just disapear), it does block however.

Any ideas how to solve this, or how to get LibreOfficePortable blocking in the background ?