I'm sorry that QU doesn't have what you want at the moment.  Maybe Mike will see this and come up with something if feasible.

In the meantime, perhaps an idea like this might work for you.
I just tried it over a white image and it works.

Using your email program, make a NEW email.
Set the point size to whatever is thick enough for your project, and use SHIFT Underscore (the key to the right of the zero)
Make a nice fold line for yourself.
You control the thickness with point size.
Now I used a selective screen capture tool that lets me capture just the line  and a little surrounds.

With my "spread" in the queue, I use "F" red button and go into Freehand mode and drag my captured line on top of the spread.
In the Page Editor, I have the ability to accurately position my created line.

Maybe that idea will help
You can SAVE the Job... and recall it over and over just changing the "spread" after the saved one loads with the line.
See snaps
Fred