It would be far easier just to write your own version than to try to reverse engineer an FPGA bitstream,and with AES encryption added you have absolutly no chance of ever even being able to read the plaintext version so you wouldnt even be able to start the process.
I was thinking about Lynn's idea and it seems better to use the idea of a free bitstream for your FPGA but to have it so that they are hooked and have to come back to you because it is limited in some way and they cannot simply take the free stuff and you never see them again.Clearly its hard to make it time limited because there is no non-volatile storage on an fpga, so what i would do is make it do something to make it run slower than you could expect in the paid for version for example or as i suggested before make the program counter limited to 6 bit,anything to keep them coming back for more.