Out of pure unacademic interest I tried to using footprint for edges. To my suprise the Edge.Cuts layer wasn't supported at all - you can't change the layer of a drawing to Edge.Cuts, although all other layers (in the layer manager) can be used! You can edit the file and replace a layer name and Edge.Cuts works then, as Rene_Poschl told. However, it still doesn't work as I would expect. The idea of footprints is apparently made for pads and things which belong to them, not for general drawing. You can't select the outlines from the footprint and e.g. move them independently, it's all or nothing, if you have text or other graphics in that footprint it moves along. You would have to open the footprint in the footprint editor to edit parts of it.
So, evidently more would be needed than just turning on the Edge.Cuts support in the footprint editor.
Another way to reuse existing outlines is copypaste. At least recent nightly builds support Copy and Paste (in the context menu) so well that I can draw graphics, copy it to clipboard, close KiCad, open it, open a board and paste the graphics. I can also open a standalone pcbnew, draw in it, copy, swap to the project-KiCad pbnew window and paste (although it's seems to be a bit buggy - I had to click to unselect the copied selection first, otherwise the old clipboard content was pasted). I can select items from several layers at the same time and copypaste them.
In this way it would be possible to have an edges library in one file - all edges in one board - or in several files.
A third way, if someone really needs a manager UI, would be to code a python plugin. It's possible to open a board file, select items from there and add them to the current board. Any kind of UI window is possible with wx, although it would lack an embedded preview.