Richard,
There is no existing function that meets your requirements so I have added your suggestion to the list for future consideration.
Assuming that you email/fax your PO to your vendor, I can suggest a workaround that may be suitable. If you set the "selling units to buying units so that you are purchasing in cases and the batch PO creation can be used without user intervention, you can modify the PO report so that for such stock codes, rather than listing the case quantity and price, the unit quantity and price is displayed.