This sounds like a great candidate for an enhancement request. Submit it and i'll vote it up. :-)
I found this page on how to get serial numbers for USB drives, that appears to cover two methods, WMI calls, and API method which is harder.
In the meantime, you could create a script that could be executed on devices and capture this information and then it could be added to LANDesk inventory via registry or other methods. This would take some work though. You'd really have to need this information i think to want to go to the trouble.