Hello David, I menaged to sort it out, but not the way you suggested.
The solution i used was on data source properties for my form used to create responses, i put in ParentID property my computed property: sessionScope.nazivParent
Then I used button with simple action "Save document" (not "create response document"), and it saved just fine, as a response to parent doc.
Thank you for your help, and I would just like to say that I'm a big fan of your videos, and they helped me a lot during my continuous learning of xpages.
:)