I haven't made a cyser, but I have made a rhubarb soda; I got a good solid flavor with 2 pounds rhubarb boiled in water for ~1/2 hour, then straining out the pulp. I don't think I'd mash the rhubarb.
This will produce a thick, tart, pink, and cloudy liquid. AFAIK, there isn't any pectin in rhubarb so I'm not sure if it's possible to clarify if you want a clear cyser. I'd add that whenever you plan to add the fruit. My preference for adding fruit is to add it after primary fermentation has finished.
If you want to see my soda recipe check here: