FCX is licensed when it is purchased from the app store. You can't register the trial copy to a fully licensed copy. You'll have to redownload it after/while purchasing to license it. However IME, packaging trial copies of anything is a bad idea as once it's installed anywhere the clock starts ticking, on your packaged app as well. I know Quark 8 does this, and I'm fairly certain that most other well-packaged software will do this as well.

Your best bet is to just buy it as FCX and other app store apps are not like "traditional" apps where you can just enter a license code to unlock it, so no post-flight script or anything.