Yes, it would be difficult. But I now have an alternative, namely half-finished book manuscripts which are found in boxes under the desk, and then I can of course ... When I discover something somehow interesting for "sovereignty," I can write it directly into the manuscript where "state" and "soverignty" is discusse.It is interesting to ask how different this alternative really is, since you could also look at the Zettelkasten as a collection consisting of manuscripts that are in varying degrees unfinished.
Luhmann would not be the only one with such Zettelmanuskripts. Nabokov and Arno Scmidt used such a method as well.
In this incarnation, Luhmann technique is perhaps closer to working with Scrivener or Ulysses.