Simpler versions of such devices exist, eg HENSOLDT Cyber markets a secure VPN gateway that is built on seL4. Over the years we've been in on-and-off discussions with vendors of networking equipment, it's always a battle between those in the company who take security seriously and those resisting any rocking of the boat. Having said that, we're presently in such a discussion again and it's looking promising so far. However, it's entirely possible that someone has already done it and is not talking about it...