Authorization check based on item category on sales order (VA01 or VA02)

I want to be able to restrict authorization of users based on item category. We only want certain users to be able to select a certain item category. I know I'm going to have to check one of the userexits in MV45AFZZ. The issue I'm having is the authorization object .

The item category is field VBAP-PSTYV.

What we are going is having a item category for emergency orders. But this requires more manual steps to associate with the original order. We already have the emergency item categories defined and working (no credit check etc) so there's no reason not to have them added to the original order. The issue is its use has to be restricted so when the user selects an alternative item category it checks whether they have the authority.

Any help appreciated