We did exactly the same as this using Linux, I'm sure there are other ways but this seemed the easiest for us as we already had a working Linux image for 99% of our hardware.
We deployed the image to our spare PCs (naming them SPARE-nnn) which then put an agent on them, did a full inventory and then shut the machine down.
In our Inventory we now have a list of spare machines, complete with inventory of hardware etc, but we haven't got an Windows/Office licenses being taken up by them. It does add extra machines to your inventory, but this wasn't a problem for us as at least we maintained visability on those machines.
We did find some benefits to the system as well, we can now query our spare machines for the best spec etc for new starters or to replace old machines so we keep our running desktop/laptop estate as up to date as possible.
What you could try is using Archive Analytics. One of the things it lets you do is to archive off the old machines that aren't in use so that you can reclaim the LANDesk licenses, remove the impact they have in your software distribution and inventory activities, but still retain their records.
This is just one of the things it does, but it does mean you have full information about them, keep history of everything that has happened to them, and also put them back into the database (manually or automatically) when they are returned to activity.
Take a look at the link below for more information and let me know if you'd like to take a look.
The One-Stop Shop for LANDesk Enhancements