On the company record/Distribution Tab/Purchasing Tab - There is a check box to Check Mandatory Pre-Posting at Release of Purchase Requisition.  If this check box is selected, it indicates that the mandatory pre-posting check is performed at the time of releasing the purchase requisition.  By checking this box you can enter the PR, and it will not check for prepostings until the PR is released.  Will this help solve your issue?  

Unfortunately postponing the moment of preposting is not the solution. The Purchase Requisition (PR) on the shoporder must not be preposted, so basically the question is : is it somehow possible to exclude this PR (with system-defined Requisitioner SO) from ‘mandatory preposting’ while the setting in Posting Control / Type M100 is set to ‘mandatory preposting’ ?

I would possibly think about this slightly differently.

If you are seeking preposting so that the PO eventually takes the correct posting, why not make PO preposting mandatory and lift mandatory from the PR, and just have it set to preposting (effectively optional). 

Then to ensure that the PR’s that are not shop order related, having accounting rules set to enforce a code part demand for code parts outside of shop order costs, this could prevent incorrect postings.

If you are looking to use the mandatory preposting to enforce the use of preposting on a PR before it is released or approved, then this may require a customisation / modification to break the standard logic.