I wanted to try to give some kind of help with this and hopefully it makes some sense. First, I don't have an ECS myself, but based on what I can find online it doesn't seem to do any video generation itself. So, it should work with this mod or any others without any trouble.
The external video input pin on the cartridge port seems to have been used for two other things though. The first being the "Keyboard Component" and the second being the "System Changer".
From what I can find the external video input as used for the keyboard component, used genlocked video - meaning it was a composite video signal that was synchronized to the Color Processor Chip. If this is the case, there isn't a good solution to make it work.
The system changer also used composite video, but wasn't genlocked (so they also changed which pin on the cartridge port was used for external video). That's why the system changer only works with the Intellivision II, unless the Intellivision (all others) itself is modified. There is not a way to make this work without modifying the system changer. Since the system changer is just a 2600 that plugs into the Intellivision, with the Intellivision not really doing anything, it probably isn't worth trying to make it work.