I was indeed not very clear on what I meant.
Basically I want to construct a lookAt-matrix BUT I don't want the up vector always pointing upwards. Here's what I want to do:

I need a matrix to project a decal so you have a projection source and a target. So far so good, I can use a normal lookAt-matrix but the point is that I want to rotate the decal on the ground, i.e. my up vector is not always (0,1,0) but should rotate around the view vector based on some orientation value. Kinda hard to explain. Maybe I can make a video tomorrow

EDIT: Coplanar meaning on one plane with the view vector so that the view and up vector span a plane (for orientation = 0 this would be the Y-axis (pointing upwards))