Thanks for the suggestion, Mike!
Unfortunately, this seems like a rather problematic thing from the perspective of user intuition. Now, I suppose you know the definition of 1 lumen, but just as a reminder for others - lumens are a unit describing total luminous output of a light source. If we allowed to set lumens as units for Corona Light Material, the actual luminosity of each light emitting object with this material would have to adapt to the size of the object.
Now, what would definitely make sense is allowing to set lux, candelas or Watts per meter squared but those are obviously not so intuitive anymore, as they are not used so much in real life.