I've just released my addon which will provide you a context menu entry to "Split" a stack of items at your guild bank.
It will withdraw the items to your local inventory, as splitting at the guild bank directly is not possible. So you need some place in your inventory.
The split amount will be kept in your inventory and the base stacl will be deposited at the guild bank again.
Everything should work as the normal split stack dialog.
It is compatible with Harven's Split Stack Slider.
But moving the base stack back to your guild bank will NOT WORK if you use Harven's "multiple split" checkbox! If you do a multiple split all items will be kept in your local inventory.