Not without creating a unique diplo unit, no. That BaseGold value is tied to the unit, and can't be modified by promotions.
The upcoming Canada for VP port is going to have a unique diplo/recon hybrid unit which modifies that base gold value. Stay tuned
If that is possible, such a change would require a settings change in the DLL to allow marsh to do that, and this mod doesn't deal much with DLL changes. That would be a feature request for Infixo, HungryforFood or Gazebo to administer.
That change would also reverberate through other systems. water mills and baths would be buildable in cities adjacent to marshes then. You could clear the marsh later and have a city with a bath, but no fresh water source nearby. The current system of having 2 separate polders in the UI is rather painless I think, and messing with the attributes of marshes sounds like a pandora's box.