It's not even possible in theory. There are several reasons for that.
1. The routing of data is hardcoded into the switches and cannot be changed without physically accessing the switch. The routing table not only determines which devices may talk to which devices, but also the direction of the data flow. This means that a monitor device cannot talk to an engine because the monitor is configured only to receive data.
2. But even if they managed to get the monitor device to send data, the switch would recognize this as a device malfunction (because it's not allowed to send) and disable the port it's sending on. This is not due to security against hacking but more due to "a malfunctioning device should not be able to DOS the plane's network".
3. There are actually two networks, sending identical data for redundancy. Now guess what happens if one of the networks sends different data than the other? Right: The offending port / device gets shut down.
4. The network protocol is a modified UDP protocol (no need for TCP) which makes the network deterministic - data delivery is guaranteed within a certain timeframe. Which means, again, that you need specialized hardware to even talk to the network.
5. And even if you managed to take down both switches, there'd still be a manual override in the cockpit which allowed the pilot to steer the plane without the network.
In essence, you need pretty hefty physical access to modify the planes flight mechanics. Something you will not achieve while the plane is in the air and even very unlikely while the plane is on the ground.