Is there a way to partially writedown/reduce the value of inventory items?

We have a situation where we purchase servers for X and sell them. We also send some out for evaluation. Upon their return from these evaluations, they are no longer considered "new" and we want to expense the item (either fully to $0 or partially depending on the eval type and length) BUT we still want to retain the item in inventory so we can manage the serial numbers and shipments of evals as we do normal sales orders.

Journal entries currently maintain the status quo with what we want reflected as true assets versus expensed items, but each time we re-stock and ship the eval products, we need to do this. As well, we continually show an asset that has already been expensed. Is there a way to pick certain inventory items and change their asset value to $0 (or some lesser value) without impacting initial purchase records, etc?