Hello Edward,
The short answer is you can't.
But you can tell the system to use Standard cost and have the difference from Average posted to a Cost variance account. Creating a Filter which shows only those items where the Standard Cost <> Most recent cost is pretty straight forward. Also, you could import the Most recent cost into the Standard Cost field.
When there is no inventory available to process a shipment, stock items are sold at their most recent cost. Allowing inventory to go negative almost always requires an inventory adjustment at the end of the month, unless you are using Purchase Orders for Adagio.
Is this a regular sales inventory (where a shipment represents revenue) or a stores inventory (where a shipment represents an expense)? Adagio Inventory is designed to manage the former, not the latter, and hence accounts on that basis.
There is a full session on Inventory Costing and Pricing at this year's
Adagio Opportunity Conference.