A lot of standard blocks are tied to an instance of the Script class, which means that they can't be used at all in global custom blocks. I'll probably look into fixing the game attribute functions though as that is the most common thing that developers want to use with global custom blocks (not now though as there's an OpenFL update going on).
Until that is fixed, you can attach your custom block behavior to all scenes and just use regular custom blocks.
EDIT: mdotedot has an alternative if you're OK with using code.