Drag-and-drop made easy

Dragging and dropping nodes within trees is easy with Ext JS 4. To allow the drag-and-drop action within a tree, we need to add the TreeViewDragDrop plugin as follows:

Ext.define('TTT.view.admin.CompanyTree', {
    extend: 'Ext.tree.Panel',
    xtype: 'companytree',
    title: 'Company -> Projects -> Tasks',
    requires: ['TTT.store.CompanyTree','Ext.tree.plugin.TreeViewDragDrop'],
    store: 'CompanyTree',
    lines: true,
    rootVisible: false,
    hideHeaders: true,
    viewConfig: {
        preserveScrollOnRefresh: true,
        plugins: {
            ptype: 'treeviewdragdrop'
        }
    }, etc

This simple inclusion will enable the drag-and-drop support for your tree. You will now be able to drag-and-drop any node to a new parent. Unfortunately, this is not exactly what we need. A task node should only be allowed to drop on a project node, while a project node should only be allowed to drop on a company node. How can we restrict the drag-and-drop action...