There is a Gather Inventory task under Jobs and Tasks.  Inside the task, you can configure what specific items you want to inventory (I believe it is a delta be default, but you can go in and change this).  You could make this a task and have this run in the chain of tasks during your image sequence.

Also, it may be worth investigating/testing the InvSoln.exe file under Program files\Altiris\Altiris Agent\Agents\Inventory Agent.

If you run InvSoln.exe from a command line with a /?, you get a screen with command line options for the EXE.  One of them is /fi, which is Collect Full Inventory.  I've run it before, but not officially proven it out by checking whether the NS actually receives a full invnetory or not, but if the built in inventory task isn't an option for you for some reason, this might be worth pursuing.

Edit:

For the task, go to Manage > jobs and tasks, Right click, New > Task

Under Discovery and Inventory folder, there is a Gather Inventory task.  Create a new task with this and customize like you would for a full inventory policy.