Stereo Delay on mono track in Reaper ?

the short answer is: use readelay, it has individual panning of every delay tap

there is no mono track per se in reaper, each track can do basically whatever you want
 
Reaper is awesome! I'm still running the demo version. Have been for way too long. Buying a new car in the next few days and moving soon, but once that is over, I'm gladly and proudly going to spend the $60 or whatever it is for a license.
 
You could alternatively send the mono track to 2 sends set the wet to 100% and pan them accordingly. ReaDelay is great though. If you are looking for an incredible delay/echo check out echoboy. It's a little pricey but you will never need another echo plugin ever again