Attempting to modify any item's/block's aspects with ModTweaker commands is currently bugged. Modifying any block or items aspects (using set, add, or remove commands) has no effect.
[Here is a test script](http://pastebin.com/KbMc8smq) that modifies aspects of Stone and Cobblestone. Loading it up in a new world fails to execute any of the scripts lines, even though the script loads.
Commands are in-line with wiki information.