Even if the gov tries to arrest them for leaking classified information, they'd be near-impossible to convict following the inevitable massive public outcry.
OTOH, I'm more skeptical about the implementation side. Have you ever set up a feed between two different organizations that needs to transmit a massive amount of data in real-time? It's an immensely complicated undertaking requiring a whole team on both sides, with managers, engineers, techwriters, QA, and support staff. It seems a little unbelievable to me that one of them wouldn't spill the beans, particularly if asked to participate in a morally-questionable program.
That doesn't actually answer the parents question. What is the likelihood that a bunch of tech leaders would know about this and it wouldn't get out? That any group of people wouldn't let this out?
I agree with the latter half of your comment, but the first sentence is as powerful as saying, "The only way to keep a secret between three people is if two of them are dead."
They both sound like ominous truths, but neither is a particularly compelling argument.
Implementation side would definitely be very difficult, but you could screen the candidates for those who are sympathetic toward it. Ensure they understand how important it is that the program stays secret. Keep retention high, and perhaps even have some ongoing benefits for any employees who do churn but were part of the program. It definitely wouldn't be easy, but I think it is doable.
There's also the issue of how to hide the source code (to my knowledge, both Google and Facebook use one source repository for the whole company, to which virtually everyone has access). This can be gotten around - I'm sure that there are private repositories off to the side that you could use to build a binary - but this is yet another integration point that could be discovered. (Eg. some SRE notices that network bandwidth is high for a DC, traces it to a machine, notices the machine is consuming a large amount of CPU, gets root and runs 'top', and suddenly notices an unknown process siphoning off all data. At this point, how could they not assume their network is being attacked - because that's really what this is, the U.S. government hacking into the company's DCs - and pull the alarm? So all the SREs, abuse teams, internal security, etc. would have to be briefed, which is small compared to the whole company but still a huge surface area for a top-secret project.)
It's not about arresting them. It's about harassing them and their businesses -- which the government can very easily do.
Also, who said those kind of CEOs are champions of public freedom? Google? Microsoft? Facebook? Because they're techies they're supposed to be "on our side"?
Not to mention it's very easy for the government to pamper them, and make them feel very patriotic and whatever for contributing to this cause for the safety of the US, etc etc.