iOS WriteRoom editor feature replacement?
My suggestion is Ulysses. A pleasant, focused writing experience combined with effective document management, fast syncing and flexible export make Ulysses the first choice for writers of all kinds.

Here is some info about synchronisation with Dropbox. A quote from that post really describes the function you are searching for.


  Once your Dropbox account is linked, you can navigate through its hierarchy and choose folders to add to Ulysses’ library.


I do not have financial ties with Ulysses!