This was easier to do in the old days of analog, modulate your cameras onto unused CATV channels, and just call up that channel via an IR device.
The best way I have found to do this now is with a TV that supports discrete IR codes for HDMI inputs. Use a small PC to display 1 or more camera streams, either with a video wall application from your VMS, or via stand-alone players. Configure the PC so it does not go to sleep and is always playing video from key cameras (driveway, front door, etc.). (This will incur some bandwidth hit on your network, but should be minimal). Plug the PC into an HDMI port on your TV, and when an event occurs use an IR blaster with an API/network interface to send an input-change command to the TV.
Some TV's also support control over a network, though these APIs are usually not very easy to find, so it is hit or miss.
The biggest hurdle can be sending an IR command to your TV on event to trigger the input change. One option I've heard people have success with (but I have not yet used) is the Mini 3. I do this as part of my home automation system (Indigo, FWIW).
I have found it is best to just switch TO the input, and not try to automate the switch back to previous input, as it is unpredictable how long the viewer will want to view the cameras for.