Some years back (several Lustrum) the hot ticket for Ruger Blackhawk's, were oversize grips that extended below the Gripframe. I have a couple. Especially popular with those whom were loading the 45 Colt up to and beyond 44 Mag performance (like ME). I haven't shopped around to se if they are still being made.
One could make a set of grips that extend below the grip frame and close up like a clam shell. Take a little work, but do-able. The 1875 is nowhere near popular enough for commercial production of such an animal though.