You don't pay for licensing unless it is assigned to a network. So if it is just in inventory it wont use licensing.
When you say "organizations" do you mean org units in the Meraki dashboard? or Organizations as in different businesses?
If you are managing all these devices under different org units in the Meraki dashboard, you can claim all the unused ones in one org to keep track of inventory, and then you can move it to a different org when you need to use it. You simply un-claim it from one org and claim it in the new org. If you happen to be using a single org and just have different networks for each client/office then you can just claim them and assign them to networks as needed. (remember you wont pay licensing unless they are assigned to a network).
Note:
By keeping them in an orgs inventory it allows you to search by MAC and see the SN and other information about the appliance.