SE is even more constrained if anything, they have just 3 platforms to work with. The peak service they managed to squeeze out of that is quite something (bearing in mind all the other constraints too!).Even if you could get to the Southeastern platforms, blocking one with an EMR set would somewhat screw up Southeastern as a consequence!
The underlying issue is that St Pancras has too few platforms, and Eurostar is given far too high a proportion of them.
At the very least, there should have been measures in place to allow some of the Eurostar platforms to be dual purpose. For example, they could have had divided island platforms at either side of the Eurostar 'middle'. You'd then open or close a lockable door/gate depending on what the platform was being used for.