If you have the custom inventory as a policy in the Altiris Agent you can trigger the policy using the following vbscript.
Dim WshShell, client, swd, taskGuid
Set WshShell = WScript.CreateObject("WScript.Shell" )
set client = wscript.createobject ( "Altiris.AeXNSClient" )
set swd = client.ClientPolicyMgr.ClientAgent ( "Altiris.SWD" )
taskGuid = "{BE8F6D17-5CEF-498C-9272-1A76586E1882}"
swd.RunAdvertisement taskGuid, 1
Change the taskguid to the taskguid you have in your custom inventory policy or any other policy you want to trigger. You find the taskguid = task ID in the details of the package within the Altiris Agent.
Example:
Hope this helps.
Stefan