GreasePencil: can't copy datablock Trying to create a GreasePencil addon I already fail at the first task. It seems the GreasePencil doesn't inherit the copy() function from ID.
Where and how could I add this (as this is pretty crucial for what I try to do)?
Nope, GD copy() is not implemented.

No way to know that from python afaik, you have to look at the C code (source/blender/blenkernel/intern/library.c, id_copy() function, around line 300).