I use fake IP Addresses for those type of items(IP's that will never be in my environment). For instance, I will use 18.104.22.168 and up for something on the shelf. I also created a custom field and call it Asset_status where I track the status of an object. Like NP for Non-Production, ACT for Active, and SAL for salvaged devices. YOU could use these or add another like OTS (on the shelf), anyway you get the idea.
Yep, thats about what I expected. And I have started something similar, even used same IP, then set the node as external. We already have an 'enviroment' field for Prod, Cert, Dev, now with a 4th field for Inventory. I then use SQL to change the MachineType and Location(also Inventory).
Thanks for the feedback, something its just nice to know what others are doing so you dont feel like you are way out in left field.