Restricting PO creation to two vendors

Question

There is a business requirement to restrict the Purchase Orders that a group of four users can create and change.  They will only be allowed to create and change POs for two vendors. 
My original thought was to add these two vendors to a purchasing group, and to create a security role that restricted the users to the Purchasing Group using Authorisation M_BEST_EKG.
However this is not how SAP works.  ME21N does not check the Purchasing Group of the vendor, but the header organisation tab that contains the EKGRP field.
A consultant has suggested that the vendors could be part of their own Purchasing Organisation.  However this is overkill to change the Enterprise structure for just a couple of vendors.
I am considering adding a user exit and coding a check of the authorisation object M_BEST_EKG.
Has anybody any suggestions?

Answer 1

Implemented ZME_PROCESS_PO_CUST for definition ME_PROCESS_PO_CUST.
Interface IF_EX_ME_PROCESS_PO_CUST of class ZCL_IM_ME_PROCESS_PO_CUST
added coding in CHECK 
selected LFM1-EKGRP
then
check the user has authority for vendor with this purchasing group
    AUTHORITY-CHECK OBJECT 'M_BEST_EKG' for user sy-uname
                    ID     'ACTVT' FIELD '01'
                    ID     'EKGRP' FIELD ls_lfm1-ekgrp.
if sy-subrc <> 0.
      Message E004(ZGEN) with poheader-lifnr sy-uname.
      ch_failed = 'X'.
      return.
endif.
Hope this helps others