It probably wouldn't be on the former D&RGW. First, there's Moffat Tunnel (I'm not sure that steam engines play nicely with five-mile-long tunnels, and yes, I know it was done all the time back in the day). Second, there's Byer's Rock. I recall hearing that Steve Lee would have insisted on a current survey of the curve at Byer's Rock before trying to take 844 through there.

I'm not sure that there's anything (other than water supply) keeping it from using any other route. That might give preference to the UP Overland Route, as they have some familiarity with water supply for 844 and 3985. (For example, probably the last new steam engine water plug in the world was put in at Ogden for the UP Steam program.)