When creating a custom enchantment, the primary_items and supported_items seem to be able to take odd or unusual items in their place and still be valid (For my example I was using a Written Book), however this does NOT let the Written Book be usable in the Enchanting Table in order to give the enchantment. It works via anvil in survival, however.

Adding my test enchantment to the in_enchanting_table tag also does not effect things.

I would expect that if a Custom Enchantment specified a weird item to be a primary_item, then it should be usable in the Enchanting Table.