I found this report by Ryan @ NINet.org a few months ago and it is great.
While on the surface it doesn't look like it will give you what you need, check this report out. It brings up a list of your computers with a link to vendor's warrenty page for that specific machine and on that page, it will give you the HP product number as well. I LOVE this report.
the first link is here, that gives the general directions and history http://ninet.org/2010/03/sccm-get-dell-warranty-info/
and here is the link to the file that includes HP and IBM http://ninet.org/wp-content/uploads/downloads/2010/03/AllCompWarReport.txt
Whitefearn