Is there any reason not to run the inventory from a single machine? Harvesting info from win32 registry and disks is usually pretty quick (on my network anyway).

If all of your machines are in a domain (assuming NT systems here), you can get a list of them and attempt to harvest. Failures can be noted in a log. This takes care of the machines that are online. In the login script, add a line to log the machine name to a remote share. You can schedule the harvester to scan the share for machines that come online and inventory them.