Fetching the address from LFA1 is already for more than 10 years not the best idea as SAP has a central address management where the addresses are stored in ADRC, address in LFA1 is not as detailed and only kept there to avoid rewriting of too many old programs.
When using one-time-vendors the user is forced to enter the address in a address pop-up while creating the PO, the data will also be stored in ADRC when the PO is saved, the link is via the address number which is stored in the purchase order header table EKKO.
Before the user saves the PO the address data is probably stored in some data structure .
So best you debug the PO creation for a one-time vendor to see in which field the country is kept during runtime and then test if you can make use of this data with teh BADI