So, the short answer to the question is "No".

Cash discounts may be applied across multiple invoices with many details on each line. having the computer decide exactly how a cash discount should be applied would be next to impossible, without a connection to the "cosmic consciousness"!

If this was REALLY important to you, then you could create a GridView View to generate an importable GL batch to reallocate the discount amount back to the various expense/revenue accounts. It would be a LOT of work though.