Anyone know if it's possible to add another source to the 'get new
wallpapers' dialogue?

This page http://techbase.kde.org/index.php?ti..._Get_New_Stuff
says that is is possible but I am unable to make what it suggests
work.

There's an entry in ~/.kde/share/config/kdesktoprc that says

[KNewStuff]
ProvidersUrl=http://download.kde.org/khotnewstuff/wallpaper-
providers.xml
StandardResource=wallpaper

But any change to the value of ProvidersUrl just gets lost after I
save and then click 'Get New Wallpapers' and the value reverts back to
what is shown here. Which is rather frustrating.

This is in KDE 3.5 btw.