The following isn't directly applicable to your suggestion, but it's a reminder that an FPGA, just like a CPU, may not be doing exactly and only what you told it to do:

http://www.cl.cam.ac.uk/~sps32/sec_news.html#Backdoor

> Abstract. This paper is a short summary of the first real world detection of a backdoor in a military grade FPGA. [....] The backdoor was found to exist on the silicon itself, it was not present in any firmware loaded onto the chip. [....]