If anyone other than me is interested in this, it seems that the kerbynet binary is not available in source form.
http://www.zeroshell.net/eng/forum/viewtopic.php?p=2917&sid=a5ad0ef4a1fc6f7dcd0ecf5d4a3a2ff8
That apparently hasn’t changed in two years. So it is not possible to debug the backend for the UI (for such things, for example, as fixing the RADIUS permission for new users).
I would personally have found that useful, so I might ditch this very nice-looking system and instead build something else from scratch. Too bad. If anyone is considering answering my questions from the first post, don’t waste your time. Thanks all.