This is an extension to JSpur's suggestion about the dummy command line and second task. I'll include a picture but basically its:
1. Create Quick Delivery with everything except a Command Line.
2. Create a script task that is the exact command line you want. In the part of the commandline where you have unique info instead enter %!unique value!%.
My example script task is one line:
msiexec.exe /i "pcAnywhere Launcher.msi" PIDKEY=%!unique value!%
3. Put these tasks in a job, Quick Delivery first. The %!value!% triggers Altiris to remove Quick Run and add an input box to the New Schedule window at the bottom.
Edit: You will of course have to do a little more than just the 1 line in the script as my example will run the script from a different working directory than where the msi is downloaded to. Example was just to demonstrate %!value!%.