I do know that there are several methods to do half-automatic tempo mapping in Reaper. "Native" or using some SWS actions. There are explanations how to do it, and there even are videos explaining some. But silly me was not able to follow any of those creating a decently handy workflow.
Here is a new video which I did not yet decently check: ->
https://forum.cockos.com/showthread.php?t=204299
Now with upcoming ARA support hopefully the Tempo Map, Melodyne is fully automatically creating, will be able to be import into the Reaper tempo envelop by a single click.
-Michael