There are functions somewhere (mod/assignment/locallib.php probably). You say you are looking to use the gradebook accross all courses, do you actually require the assignment functionality or are you only interested in gradebook items? I've written a block to distribute grade items accross courses, as it stands it only works with manual grade items, but if you require assignments you could use this as a base and add that functionality rather than starting from scratch.
Mike
Gradebook
Query regarding assignments - advice needed
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