Comment 3 for bug 2006752

@Luke : Did it work ? I am also trying to log stderror along with stdout when using in-target with d-i.
So, far I've only found that stderr is not provided by in-target, we can only direct stdout using the --pass-stdout flag.

Although, there is an interesting solution here, have a look:
http://www.virtualbox.org/svn/vbox/trunk/src/VBox/Main/UnattendedTemplates/debian_postinstall.sh