I have a very poor quality picture showing the pipe from the right side water leg on OY capped off and the aux pipe right next to it. This seems to add more credit to that thought.
This is a close up of the Aux pipe and tender connections on the RGS rotary. They have the gladhand connections instead of the threded unions the D&RGW Plows have. It shows the tender jooked up and the aux disconnected. I have another picture of the RGS No2 sitting in the same position, I think on the same day that shows it does not have the aux water tender. The RGS did not always use one with the rotary. Since they ran their aux tenders for the rotary the normal direction, they could use any of their ex tender water cars but used the tender from engine #2 for years. It had a shack appearing wooden roof over the coal area and apron. The frame must have rotted out so they mounted the tank and roof on flat car 7451. The flat was cut down from an ex C&S stock car and made an interesting "quaint" looking car. They also used the tender from #12 (W12) a lot. This water car was renumbered 019 at the end.
The OM and OY had ex tenders modified in 1935 for aux use and rarely are seen with out them until replaced with the ex tank cars in 1958.. They lost any number and were lettered for their rotary. OM's had the brake retainer moved a brake wheel installed along with a couple instead of drawbar. It had a wheel set mounted on the front porch, probably to offset the lack of coal weight. OY had similar modifications to the coupler and brake wheel but had a peaked wooden roof installed over the coal bin and a sheet metal front. I have to assume that the piping came from the left water leg (now on the right because the car was run backwards) under the car and came out under the end sill right behind the tenders aux pipe.