Hi, Wen,
Seems to me you have a couple of choices -- the first would be to call ahead and book a room at the Casa Rural, El Puente is its name (I think). It is a small place, maybe only 4 or 5 rooms, and was full when I called in early October several years ago, but that was probably because it was on a weekend. I imagine that weekdays in September won't have as much traffic as weekends.
the other option is to just show up there and if there's no room, see if the owner will call you a taxi from Melide. It will cost about 1 euro a km and you can then go into Melide for the night, taxi back to Ferreira in the morning and continue walking (or just skip the Ferreira to Melide part, depending on how much of a "purist" you are -- but remembering that you won't qualify for the compostela unless you walk it, if that's important to you).
Or you could decide to carry a sleeping bag and try to sleep at the very lovely albergue in San Roman.
buen camino, Laurie