What is it that you are trying to accomplish by including a refugium for freshwater? Generally speaking, in the saltwater world were refugiums are far more common, the purpose of a refugium is two fold; to export nutrients through algae growth and removal, and to provide a safe refuge away from the main tank and potential predators for maintaining a population of healthy invertes that can be redristributed over time into the main tank as a supplemental food source.
If nutrients export is part of the goal, then I would think that fast growing low tech plants that suck up lots of nutrients would be desirable. Things like Elodea, duckweed and water lettuce for example. None of those plants require any type of substrate. If supplemental food is the goal, most people are not using RCS as supplemental food, so I'm not sure why you would include them. Something like fairy shrimp, scuds, or freshwater pods
would seem to be far more viable.
If on the other hand, your goal is simply a supplemental safe haven to raise RCS, then that is an entirely different question.