Following the advice of others, I made the following modifications:
Used five cups of rhubarb instead of four; cut the nutmeg in half and added a quarter teaspoon of cinnamon to the topping.
For those of you who are wondering about using frozen rhubarb, I did -- but I thawed it first. I drained off all but about a quarter cup of the liquid that was created as it thawed. This helped with the stiffness of the dough in the final stage and didn't seem to affect the finished product at all. From what others are saying, my batter was probably a lot easier to spread in the pan.
As for results: I took the cake to my parents' house on a visit and everyone there really liked it -- even my dad, who isn't the easiest guy to please!
I will definitely make this again; the recipe is going into my file. Thanks, Deb!