Oddly enough, pushing images to an FTP server on iOS is easier than pushing a plain text file. Many text editors for iOS only share the text. Few apps share an actual file object. 1Writer does this perfectly and makes pushing text files to Transmit dead simple.
From 1Writer, just share the file as normal.
Share to Transmit or DEVONthink or whatever app you want to accept the text file. In my case, I want to push the file up to an FTP server using Transmit.
Textastic is also an excellent text editor and shares a file object instead of just text. I prefer the look and feel of 1Writer.