
You could try using the USB as the multiview, going into a laptop and setting up the monitor as a second screen to the laptop. Then place whatever software you use to view the feed (Quicktime Player for example) on that second screen. That way the ATEM won't know it's ending up on a 60hz screen and you can have your ATEM settings at whatever you like. If you already use a laptop via USB for software control then you can still get the video feed over the same cable and still have one spare for recording.
If you have to feed more than one monitor though then this probably won't be the solution you're after. One thing I found before I got a monitor that could accept 25p was that if I put an old Lilliput camera monitor in the way between the 50/60hz monitor and the ATEM (very old cheap SD one that I used to use on a Canon 550D) then it worked - I guess the HDMI out port on it could scale it out to 50/60hz. So if you have any old camera monitors lying around that you think are past their prime, try using them as a converter and see what happens.