It'll be a good challenge...NOT! FBA is still gonna be open source. i.e. You're still gonna make it too easy. It may move past random packet sniffer's ability but since the way things will be initiated will be public, whoever is hacking will be after the special parameters you put into whatever package/algorithms you use which you did not make open source. i.e. just a matter of spending a few hours with the disassembler. You should know by now that the word "safe" is relative. It's never "safe"...just "safer." + The unneeded overhead of the setups and encryption.
In short, if an illegal build really wanted to have itself included, you could do little to prevent it.
+ There arent a lot of illegal builds around anyway. The only ones I can see is the version used by GGPO and the version used by 2df. Both of them have stopped updating a long time ago. I strongly doubt that they're gonna upgrade to the latest versions of FBA unless something really good...like say Naomi for example...is added to FBA. I can't imagine them being crazy for tiles and previews either.