Hi, I know this is old, but I searched for "outlinerEdMenuUserCallback" and found only this related post.
I just managed to put some custom items on the Outliner right-click menu.
From what I searched (and witnessed by reading the OutlinerEdMenu.mel), the only thing this script does is to source another script named "outlinerEdMenuUserCallback".
And that's what I did. I created a script in Maya's Script Path named "outlinerEdMenuUserCallback" and put my menu initialization there:
global string $gOutlinerPopupMenu;
menuItem -l "MY_LABEL" -c "python(\"import myModule; myModule.myCmd()\")" -p $gOutlinerPopupMenu;
So everytime the Outliner is first created, my custom script get loaded and adds stuff to the Outliner menu.
Hope it helps.
Richard