Just to be sure, it is OK if I modify a footprint manually (after drawing in some unused layer in the GUI) to change that layer in the editor to Edge.Cut for a closed contour. Let us assume that I specify a line width of 1 mil. I say then: 0.0254 for the width since the footprint seems specified in mm.I must have to save that footprint in a special place since it will be overwritten if I update.
So then assuming I choose that footprint I can forever more use it and count on getting a closed contour edge cut in my board where I want it, as long as I never make the mistake of editing that footprint in the GUI footprint editor. Until version 6 when it will become allowed.
Thats pretty much right, at least to my knowledge.
One thing I would do differently is not edit the line width manually. Instead draw it in fp editor on eco layer with the line width you want and then only change the layer in text editor.
That footprint will be very annoying to use. Every time you try to select something the edge footprint is there if it has all the board footprint because it covers the whole board area. In 5.99 this is easier because you can ignore locked items while selecting.