Taco Hoekwater wrote:

Is it possible to make this work? (maybe an extra \chapter command 
before the combination that triggers the header-footer setup, but that 
is not printed after all?)


I would put the externalfigure in the before= of the chapter head,
maybe wrapped in some form of \doifelse.

I managed to get it working, thanks again Taco. (Once this project is finished I'll add stuff like this to the wiki.)

Groeten,

Jelle