I don't know the scale of the problem: for example is this for a "large" research lab that just got an "intelligence" contract and now has to "do something" about security or is it to protect the computers of the rest of the family from grandma's laptop (which has proved to be something of a virus magnet) when she comes to visit and plugs it into any RJ45 socket in the house that seems good at the time.
The details of the requirements seem to be gradually unfolding. Lets see if I can flush out a few more.
Here's a couple of other ideas to consider:
Disconnect the cables (apart from cables to sockets you can physically secure or are always "insecure") and require the mixed cases to use encrypted wireless with multiple wireless networks (at least "trusted" and "untrusted").
Use VPNs (Virtual Private Networks) on the cable. The VPN encryption isolates the "networks".
If either of these are ideas could be useful (and I can think of a number of reasons why they might be "less than ideal") then I think you would probably need to provide further information along the lines of the number of computers involved, frequency at which computers change from "trusted" to "non trusted" (and vice versa), distances between computers, intervening materials, bandwidth involved, communication patterns, budget etc because neither of these ideas would scale as well as the other "less than ideal" proposals on the table.
Some more questions: what makes a computer "trusted" or "untrusted"? Location ("untrusted" in an insecure area because someone passing by could see "sensitive" information on the screen)? the computer's function ("we don't trust the computers controlling the dishwashers because the software comes from North Korea")? the computer's user ("Grandma has an uncanny knack of downloading the most active and vicious viruses")? something else? And what causes a computer to change from "trusted" to "untrusted" or "untrusted" to "trusted"?
I've been a bit light hearted here partly because I confess a growing suspicion that making minimal changes is a higher priority than implementing any genuine network isolation and if that is the case I'm probably not very effectively using my time if I make any further contributions to this topic. My apologies if that suspicion is unjust.