Here is a link to a small mod that will allow you to test the summons in custom battles.
https://drive.google.com/file/d/1ij-ov3sGFACZdSC39MnfTQAVoc__JoXV/view?usp=sharing

Install instruction, just simply place it in your Total War WARHAMMER II\data directory, requires the main mod is also activated.

Note this is for testing only, the units in customs do not represent what is available to each faction, they also cost 0, playing a campaign with this enabled will also add all the Binding scrolls to the Beastmen culture and also all of Kinship scrolls to Bretonnian culture,