I think Nelson's use of a more general graph has some value. I could see an encyclopedia or a program being edited/viewed in Gingko with circular references. With some tweaks on the UI (being able to put more emphasis on the card currently viewed, maybe by partially sliding out of view the parent cards or shrinking them when they're not focused).

I think this project is pretty cool and the idea is awesome. It would make a lot of sense on a tablet/phone and as a generalized editor.

Zig Zag was meant to represent a very general set of data structures, if you're interested in documents then you, probably, would have a more specific interest in his (very long running) Xanadu project - https://en.wikipedia.org/wiki/Project_Xanadu - which in some ways is very similar to OP's stuff - http://youtu.be/En_2T7KH6RA?t=3m27s