let me start by telling you that I bought editorial some time ago and I felt in love with it. Fantastic!
But, like the initial comment, I do also miss keyboard customisations. At the moment, I'm doing my writing mostly in English, but I'm not a native English speaker. My mother language is Portuguese, and we do use a lot of accents, like à, é, ã, õ, which take a considerable amount of time to type in iPad's keyboard.
Like Helgi mentioned, iA Writer does that pretty well, and it works almost like a normal keyboard, where you press the accent first and then the letter.
I do believe that the main purpose of the keyboard extension in Editorial is to facilitate the usage of markdown, and I don't know how hard it would be to, maybe, offer another keyboard extension, one that you could switch to when needed, but it would be a very nice addition. Right now, when writing in my own language, I have to switch to another application, since putting accents in Editorial brakes my writing flow. Besides that, it is a fantastic writing tool.