After much persistence, I finally found an obscure article which proves rebar was around and definitely used prior to the Revolutionary War both in structures and statues. It was mentioned that the George III statue in NY was pulled down and melted into lead shot for the colonials, but there wasn't tons of lead produced. Only one gentleman pointed out the fact that lead (and even bronze) statues had some form of inner skeleton and it was usually iron rebar.
Extrapolating on this, I still hold to the premise that this giant grapple indeed dates to the late 18th/early 19th century, with perhaps the latest date being around 1850-ish. I know that today's grapples are also made of common rebar, but I'm sure this is simply a much earlier practice that just held on through the years. This is an interesting article about the difficulties of saving these old lead statues as the rebar eventually rusts away.