You can use the customer editor to allow custom payment methods for specific customers. This can be used to enable methods like "Purchase Order" only for customers who are approved to use them.
1. Create the custom payment method.
Settings > Payments & Taxes > Custom Payment Types
Note: The "Admin Only" box should remain unchecked.
2. Mark the custom payment method inactive under Settings > Payments & Taxes > General.
(If the method is Active for the whole store then you will not be able to assign to specific customers.)
3. Pull up the customer record you want to assign the custom method to and check the enable box. Customers > Edit Customer