There doesn't seem to be a way to change the icon for the folderview widget (list view) when it is in a panel. Therefore you have to change the folder icon before you drag the folder to the panel if you want a special icon. But this does not always work, especially if the folder is a subfolder of a folder in your home. In that case the icon always shows the user-home icon, no matter which icon is assigned to the folder in dolphin. It does not happen with all folders, maybe you nonetheless have an idea... It does not depend on how long the folder has its icon and it does not depend on the folder the subfolder is in... I' using KDE SC 4.12.4, but this is also the case in KDE SC 4.8.4 as a friend told me. Reproducible: Sometimes Expected Results: The folderview icon in the panel should either always show the individual icon of the folder or it should be possible to configure the folderview panel icon.
As this is a little old and the folder view since Plasma 5.4 allows to change the icon I'll mark this as "resolved".