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.

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.