Originally Posted by
Hypnos
The person captured would either qualify as a civilian (and thus fall under the regular GC rules for such people) or an un-uniformed solider (and thus fall under those rules). There aren't really any unaccounted sets in the GCs.
Aside from that, you don't really get the propaganda side. Once you stop playing by those rules, word will get out sooner or later and at that point all the goodwill you've built up by your public actions is null and void. Moreso than if you'd just been forthright from the beginning. And that mud sticks. It completely undermines the PR work from the beginning and the precedent it sets will haunt you well into the next conflict. After all, why should they believe anything you say after getting caught in a lie that big?
As an aside, Gitmo is an attempt to do an end-run around both the international conventions and federal law. Quite a succesful one at that, but it is essentially an extra-legal system.