If you can find a registry key that differs between the trial and paid for version, you can create 2 different software resources with those detection checks.
In the paid for app resource, have it require the paid-for registry key.
In the trial app resource, have it require the trial key and also make sure that it looks for "NOT paid for key".
the latter rule should cover any issues with someone having the trial version and upgrading to paid for, assuming that would keep the trail key in the registry.