How can I select channel index/root pages from an Entries Field Type?
Having thought about it a bit more, this doesn't really make much sense. Obviously the entries will only link to... entries. I got a little confused as to why it also allowed me to select "single" pages. 

I still think it's worth including the option to add channel index/root pages. In the meantime, because I'm using this as a crude navigation maker, I've just created an additional single section. I then directed the Entry Template to the same directory as the index.html page. This then gives me the option to add it in my entry field, without messing any links up.