I have a similar need to control ATEM(s) -- and have looked deeper into it.
The official ATEM SDK doesn't document the TCP/IP or UDP calls for controlling the mixer directly through ethernet. Instead it provides the C++ libraries one needs to write a max external to do the job. Therefore one will need to volunteer to write the external, or I personally would be interested in paying for this object. I will post to the DEV section of the forums and see if there is any interest.
meanwhile, there has been some effort in other software, hardware platforms to make cheap controllers or custom software. You can sift through the atemuser site more info http://atemuser.com/
I am interested in controlling my ATEM switcher and tested ofAtem, but it doesn't seem to work with the current version of the Blackmagic software. I contacted the softer designer but he doesn't respond. Anyone else have this working?
I have used both ofATEM and atemOSC and prefer the latter for more control with my ATEM Television Studio. https://www.buechele.cc/blog/2013/2/5/atemosc-a-osc-bridge-to-control-atem-switchers. It works great with Max. No problems at all for the past couple of years.