Hi Dan,
They absolutely made Reithosen and Stiefelhosen without piping. Regular infantry Tuchhosen with red piping were made right up until the end of the war, though the feldgrau cloth was replaced by a gray color called "graue Grundtuch" for a period of time. The feldgrau color eventually returned in 1917. There is no mention in Kraus of the piping ever being dropped. I think most of the infantry style trousers you see today that are without piping either had it at one time and it was removed (I have seen examples of this) or are postwar trousers that never had any piping.
Chip
