How can I attach a curtain rail to a lath and plaster wall?
Finding studs behind lath-and-plaster walls can be difficult because the plaster is so dense. See Am I having trouble finding studs because my walls are lath and plaster? for some suggestions.

However, since you know exactly where the strip of wood has to go, you should be able to drill some small pilot holes behind its eventual location, then fish some wire into the wall cavity to find the studs on either side of each hole. I wouldn't count on having all the studs at 16" centers, and you may even have some studs doubled up, so I'd do more than one pilot hole to be sure to find enough mount points to attach your curtain rail securely.