After a bit of more searching, it seems as though the only option is to use a pre-install batch file combined with errorlevels and if exist commands.
That works well enough, although I have a minor gripe / feature request -- using this method will always result in the deployment showing an ERROR/FAILED for any computer that already has the software. It would be a little nicer and cleaner if DC was able to report 'already installed'. It makes reports look a lot cleaner, too. My boss logged in for the first time earlier today and was concerned there was so many deployment errors, despite the fact it's benign.