Switching between wallpapers still works (minus the usual glitches of default values not being applied but that's a different story) and applying settings. I can now switch between Hunyango and Haenau no problem.
Ideally this dialog was refactored to use onActivated instead of onCurrentIndexChanged and Component.onCompleted to populate itself etc etc but when I did something similar to applet config dialog, it broke various places relying on the fact that values change *after* creation, so let's go with this least invasive patch, for 5.8 series anyway.