MIRO: Field Status control per GL master for non-goods receipt invoices

Question

When posting incoming invoices in FI using FB60, the combination of GL account and CO account assignment are validated and checked against Field Status Group (FSTAG).  Such controls are also in place in MM for purchase requisition (ME51N) and purchase order (ME21N), in addition to checking the account assignment category.
But there is problem in posting non-goods receipt (2-way match) invoices in MIRO u2013 system checks GL account and account assignment object according to account assignment category in PO, but not the field status based on GL account.  GL account and acct assignment object are proposed by purchase order.  However they can be overwritten by users with no validation, often resulting in incorrect posting.  Wonder how other manage this issues in their companies.
I would like and need help in implementing a validation or user exit in MIRO which will check GL account against its CO account assignment for cost centre, order or WBS, based on the field status group of the GL master.
Q1,  Where is the user-exit or validation maintained that checks the field status group in FB60 and ME21N (not in OB28 or GGB0)?
Q2.  If purchase requisition and purchase order  check field status group based on GL account, how can we make such controls available in MIRO? 
Q3.  What is the transaction code for developing a validation for MIRO?  (MM MIRO not appeared in the application area of GGB0.)
Appreciated any help

Answer 1

Hi
Th evalidation should be at the source is the Purchase Order. I beleive it is standard SAP Config to put the validation there. So once the PO has been created with the correct account assignments the GR and IR will be correct.