Modifying behaviour of "Stationery Pad" function in Finder
If, instead of marking the file as a stationary pad, you just lock it, most software will open it read-only, and you can just do a save-as, accomplishing exactly what you want (An open file that is a duplicate of the locked file and hasn't yet been saved.) The file will be write-protected.

Unfortunately, the stationary pad feature is designed the way it is and (as far as I know) there isn't a workaround to change that. Most word processing and other software already has a built-in template feature, so that's worth looking into, as well.