Thanks, I’m unstuck now! Your answer in Transform: scale - #6 by timeanddoctor is useful, too for folks in a similar situation, just remember to remove the alpha channel before loading the picture (I used gimp). Then register by moving the 3D volume. The picture allows one rotation and three translational degrees of freedom. Another hack I guess would be to let a reformat widget track the picture if it’s subjected to all 6 degrees of freedom, but your suggestion will do for now!
Also you taught me how to vote for a github issue – that somehow had eluded me until now.