Just to clarify, I'd like to think I have the skills to implement this myself - but I don't want to maintain a fork of Dokuwiki! Hence the feature request here is for the purpose of:
1) identifying functionality which might be of use to other people as well as myself
2) see if it's possible to do this via a plugin (i.e. not changing the core code)
3) if I did produce a patch, would it be likely to be adopted
Of course if someone else wants to do all the hard work then all the better :).
Even better, if it's possible to acheive the desired result using the current code - I just came across this:
And looking at the admin page: "This might result in hiding of accessable subnamespaces" - seems to do most of what I want - but there is a risk that someone will consider this behaviour a defect which needs fixing. I need to do a bit of testing to confirm.