Adding Applications after Imaging

Make a Smart Computer Group that gathers the Macs together that do not have the specific application present in their inventory, like so:

Software Information > Application Title > does not have > "Application name"

If you need to, add in any extra criteria you may need to get the exact Macs that you're targeting, such as Location info or whatever.
Scope that Smart Computer Group to a policy that will install the missing application using an every15 trigger (or whatever your check in interval is) and a Once per Computer frequency. It will install silently on the Macs that need it when they next check in.

You could also force deploy it via Casper Remote and use the same Smart Computer Group as your target systems.

Lately I've been using smart groups which look for package receipts rather than application titles. The advantage of this is that when there is a new version of the application, say Firefox 11 becomes Firefox 12, the application title doesn't change but my installation package name does. I can change the smart group to look for the new receipt and set the policy to install the new package and presto, all the machines which have a receipt for firefox11.pkg but not firefox12.pkg jump into the group and trigger the policy which installs the new version then updates inventory at which point the machines drops back out of the smart group. Easy and painless, no messing around with application titles and version numbers. once I have the package built it only takes a few minutes to get it into the JSS and change the policy and smart group criteria.

Cheers

Lincoln