I've been thinking about this one a little more and I've got a few comments to add.
1) The issue with adding them to the database is the size of the environment. If I am adding hundreds of nodes at once I risk impacting the performance of the Orion server with elements that I am not going to monitor. When you are hosting environments with thousands of nodes, every bit of performance impact matters!
2) If I could flag the nodes into unmanage mode on entry, this would allow me to run a custom report against all unmanaged nodes so that I could pull the resource details and get them approved by the client.
3) All of this would be solved if there was a) an tool outside of NPM to provide the resource details of a list of IP addresses and b) a way to import a list of nodes and resources from that said tool back into NPM.
It seems that Orion has been developed for the 'subnet sweep and monitor' crowd but is missing the ability to be granular about what resources you want to monitor without some significant manual intervention via the GUI.
Just thinking as I bang my head against this problem this afternoon.