After consulting with several people, including bike mechanics there were two possible solutions to cleaning out this radiator. This gunk seems to be a very hard mixture of asphalt and sand. It doesn't come off easily and is the consistency and hardness of concrete. The only thing that partially softens it is penetrating oil. The dark spot on the left side of the picture (bottom of the radiator) is my little test patch. Success was limited. I tried gasoline as a solvent but had no luck with that.
So, my next option is to make radiator soup. I will take the temperature sensors and rubber pieces off and boil the radiator in water for a while. It can't harm the radiator and should soften the glommy mess. We shall see.