External purchasing system integration with AX 2012

AX gurus,

I am leading an AX 2012 implementation, currently scoping the customization and integration effort. AX will be financials only (GL / AP / AR - no inventory or related).

What are your thoughts and experience on the possible options for the vendor invoice match process when the purchasing system is external?

As-Is process:

1. We have systems that handle both master data and transactions for everything related to inventory, supply chain, and production
2. We run SAP and duplicate all of the master data and transactions related to inventory including PO and receipts activity there, mainly for the purpose of getting the financial amounts
3. This is not an efficient solution as the systems don't tend to agree and there is much reconciliation work. It is painful.

A primary AX goal is to avoid any inventory-related setup or data and have it receive the periodic, summarized financial effect of the external systems' activity, treating them as sub ledgers.

Options that I see for invoice matching:

1. Build a custom form to do the vendor invoice entry and matching process (either in AX or outside it), then submit the complete and matched invoice into AX using business connector or AIF
   1. Is it feasible to send in the external PO and receipt reference numbers on the invoice such that they will be accepted by AX as-is without a corresponding referential record?
2. Modify the existing x++ code in the vendor invoice (VendEditInvoice) form and related matching screens to load and substitute the external data instead of the AX internal PO and receipt data
   1. This seems like a big customization effort that would be infeasible and present a significant project risk

Our sister company has a similar system structure. Their solution was to set up AX inventory and bring in the PO and receipt from the external system at the point of first receipt, and thereafter use the matching in the standard way. For our needs this would not be ideal as it puts us back to a similar pain point with duplication of data, and our volume is many times theirs.

Any input or ideas is appreciated.

Thanks,
Paul