Crawl out on that limb?

To speculate means, basically, to make an educated guess. Speculative means guessing rather than knowing. To go out on a limb means to expose oneself by taking a position which is not certain. So, Julius is saying that he doesn't know if the machinery can be used to respond to cognitive irritants, and that it would be speculative to say it can. Therefore, Julius is not prepared to expose himself to the risk of being proven wrong by saying it would work when he doesn't know for certain that it will, he is not willing to go out on a limb.