Do RCDs work for DC systems?
Standard RCDs only detect AC, as they are based around a transformer.  The live and neutral both pass through a sense coil.  Provided that the currents in each are equal and opposite, their magnetic fields cancel out, and the sense coil detects nothing.  If current leaks from the live to the earth, then the live current is greater than the neutral, and there's an imbalance in the magnetic fields.  The sense coil produces a signal, and the RCD trips.

What are you actually trying to protect against here?  24V DC is unlikely to electrocute anybody.