Hi Sally,
I found a solution, but it's essentially the same as your approach. On Symc support site there's a HOWTO with some sample tasks - one of which is a VBScript that will do token replacement on an arbitrary text file (see http://www.symantec.com/business/support/index?page=content&id=HOWTO44396&key=55263&actp=LIST).
I push my image with a custom unattend.xml sysprep file. I replaced @compName with %SERIALNUMBER%. After pushing an image (and whilst still in automation before reboot), I execute the VBScript against c:\windows\panther\unattend.xml which replaces the %SERIALNUMBER% with the Altiris generated token. Then do the reboot, let Win 7 do it's setup and viola - computer ID is the serial number.
AFAIK from reading the forum, this seems to be the way most people are doing it, but I think there's no real difference from the way you've found.
Anyway, thought I'd post my findings.
Rich