Answering parts of my questions myself:

Adding a custom widget to KConfigDialog requires
1) adding a new entry to the QSqlPropertyMap
2) registering the signal which is emitted when the custom 
widget's value changes

I'm stuck in point 2...

KConfigDialogManager documentation:
---
If you add a new widget to the QSqlPropertyMap and wish to be 
notified when it is modified you should add its signal using 
addWidgetChangedSignal()
---
But where is this function? The only addWidgetChangedSignal() I 
found is in kautoconfig.h, which is not part of KDE. Does this 
mean there is no easy way (without subclassing one of the 
KConfig* classes) to add an entry to
KConfigDialogManager::changedMap ?
 
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