To be honest I have no idea how KConfigXT works but from what I see it is an xml file and a widget that can operate on it. If that is the case then creating a model out of the settings shouldn't be a problem and then you can use a mapper with it. Also storing a list in the file itself shouldn't be a problem, I'm sure it supports whatever QSettings supports and more and storing hierarchies (using groups or arrays) with QSettings is a usual thing to do.

Just to make sure - we are talking KDE4 here, right?