Ok, I think I have something here!
Hopefully the screenshots are self explained, but in the result image, the 'True' points in green are the ones from the initial array, the blue 'True' ones beneath are the ones in the extended array. I'm basically using the fact that we know for sure that there's only 2 values in an Edge Vertex array, then working from there. It's probably not the fastest thing in the world, but it was the only way I could work out.
To be honest, this would be 10x easier if there was a 'is a value in this array also in that one' node. But there isn't, alas. :)