As bowlds suggested, it would certainly depend on what fabric you plan to use. Also the designs, as tearaway would come off better around the edges of a very filled design than a really sketchy one with gaps everywhere. If you were to use a cotton type fabric you could line them to hide the back and help insulate if your climate requires it. If they are to be thin fabric and see-through, it sounds like a job for the WWS (which is washaway anyway?). I wonder if you could even do them like an applique and place some fabric behind the design before the final outline -then trim close to the edges? Again, it might depend on the designs you use as well as your fabric. I hope you have some spare fabric and you could have a play with different methods first.
I would not use plan tear away myself because usually curtain fabric is thin and you would see the bits left behind. I think washaway would be better if the fabric is thin.