It would be nice if folders natively supported icons.
Proposed syntax:

folder('my folder') {
bundledIcon('package')
iconUrl('http://url/to/icon.png')
}

Relevant configure blocks:
bundled icon:
configure { folder ->
folder / icon(class:'com.cloudbees.hudson.plugins.folder.icons.BuiltinFolderIcon')

{ baseName('package') }

}

icon url:
configure { folder ->
folder / icon(class:'com.cloudbees.hudson.plugins.folder.icons.UrlFolderIcon')

{ url('http://url/to/icon.png') }

}