I have written a patch that allows nested targets for drag and drop. The nested targets can accept different source types. I ran the test suite but I don't think that the dnd test suite is included in the default setup.

Index: dnd/Manager.js
===================================================================
--- dnd/Manager.js (revision 12205)
+++ dnd/Manager.js (working copy)
@@ -14,6 +14,9 @@
this.target = null;
this.canDropFlag = false;
this.events = [];
+ this.nestedTargets = false;
+ this.sources = new dojo.NodeList();
+ this.leftSource = false;
},