I added IN and LIKE to the attribute filter operators in node fetching. I use it in a form where user can click one or more status checkboxes to filter the display of TODO items. The IN filter expects an array like (1,3,5) which you have to provide in the template and is actually passed as extra parameters to the templates in my implementation.
I think this can be useful for you too, I'll upload the modified class ezcontentobjecttreenode to pubsvn later today.
I patched against rev 7077 of the trunk. If you inspect the diffs, you see it is all very easy to do.
To feed an 'in' array, I usually assemble this array as a string from checkboxes inside a form. The form lets the user specify some nifty filters in a trouble ticket like interface.
If you like it, you can fire another suggestion to the ez crew to incorporate both filter options in the standard code base (I already did without success).