Yes, this would be very doable. As you say the tricky part is populating the "LeaseExpire" value into the registry. Actually if you have a spreadsheet with the serial number and lease expiration dates, you could automate that part as well. The script would run, capture the serial number from WMI, then look that up in the spreadsheet file. Optionally if the SN was not found in WMI or the spreadhsheet, possibly it could prompt the user to enter the value in a text box manually (which would then be written to the registry) or to call the Helpdesk for the information. Then the LeaseExpiration date could be written to the registry, where it would then be captured by a simple custom inventory.
When the custom inventory runs, it generates an NSI (Notification Server Inventory) file. That file gets wrapped up with other Inventory .NSI files and sent to the NS where it is loaded into the DB. So your custom inventory would result in a table (maybe Inv_AeX_Lease_Expiration) which would have two columns: _ResourceGuid (which is the Altiris-assigned identifier for each computer and is unique) and the LeaseExpire value. The resulting report then would use SQL statements to JOIN that table to other tables which have "meaningful" information, like the vComputer view in the database, which has computer name, primary user, OS info, etc.