Just a bit of side information as I talk to a lot of the users of "Edit Order" and people are really doing some cool stuff with it now they have access to edit custom properties. Some use cases we didn't consider but are helping people manage their stores.
Custom Properties as Back Orders - This is actually a really good use case, some users are using the Custom Properties to add a "Expected Delivery" field and then the date as the value. Customers can see this in the customer area and automated system emails.
Custom Properties as Partial Fulfillment - Some people instead of splitting products as different line items are simply using a custom field called "Qty Fulfilled" and putting the current number in as the value.
Custom Properties as line item specific Notes - Some users are using the custom properties to put additional information on their invoices and picking slips for a variety of reasons such as a custom field called "NOTE:" and then a value of "Customer has asked if they can receive the bunch with the most white flowers"
Custom Properties as markdown notes - I've talked to a few people who when editting the order and marking an item down are putting the original price as a custom property such as "Original Price" : "$19.99" and then "Discount Reason" : "Price matching a competitor"
If anyone has any more good idea's for using the custom properties let me know as I always love seeing what people can do with something so simple.