External tax calculation during requisition

Our client requires the SAP PR line item to record a tax value calcualted from Taxware. This estimated tax value will be used for a more accurate approval routing (This is an R&D shop therefore there are significant budget constaints thus requiring their approvals ot be exact). In my experience SAP would call the external tax engine at the time of PO / invoice but this is unacceptable to the client. Has anyone had any experience calculating tax externally from the SAP purchase requisition? Where would we store the tax information in the PR line ?

Possible workaround we are discussing is to create a background PO that would call taxware. This PO would send the tax value back to the requisition.

Any suggestions would be appreciated.

Thansk,

Dave