I did some research on that. The Coulterville Road was only constructed as far as Crane Flat when McLean "discovered" the Merced Grove. The 6 miles from Hazel Green to Crane Flat that had been constructed were abandoned and the route was switched to dip southeast from Hazel Green to the Merced Grove and thence on down to Big Meadow. Apparently, the original plan of the Coulterville Road franchise was for their road to join the Big Oak Flat Road at Crane Flat and then share the same route from that point to the Valley via Gin Flat, Tamarack Flat, and Gentry's. A road from Crane Flat south down the Crane Creek drainage to Big Meadow was not contemplated.
In fact, the road in question was not built until 1914, and it was built by A.R. Davis (the road would become known as the Davis Cutoff), who had a ranch in Big Meadow and was keen to take advantage of (1) the railroad terminus in El Portal and (2) the new rules allowing automobiles to enter the park. His cutoff road, coupled with the new steep road built by J.R. Wilson, an auto stage operator, from El Portal up to Foresta, would allow visitors a more direct routing to reach the Tuolumne Grove. Coupled with the rehabilitated, formerly abandoned section of the original Coulterville Road from Crane Flat to Hazel Green, travelers could make a loop from the railroad terminus at El Portal, allowing them to see both the Tuolumne and Merced Groves.
Apparently few people took Davis up on his offers and his toll road cutoff did not turn a profit. It was eventually abandoned and acquired by the park. I don't think it was ever paved, unlike the Coulterville Road. It was kept open for auto travel until sometime in the mid-to-late 1930s, when it was closed (probably in preparation for the imminent opening of the New Big Oak Flat Road from Crane Flat to the Valley in 1940). The old connector from Hazel Green to Crane Flat, however, was kept open to automobile traffic by Mariposa County up until the time when this area was added to the park in the 1950s. The construction of the modern New Big Oak Flat Entrance Road in 1966 was laid over a large portion of this route, except that the new highway turns north to Hodgdon Meadow rather than continuing west to Hazel Green. When this road opened, a new unpaved road was built to connect it to the Old Coulterville Road above Merced Grove (this is the road one now walks along from the Merced Grove trailhead to the point where it meets the Old Coulterville Road at a switchback, which you follow downhill to the grove). At some point, the Old Coulterville Road within the park was closed to auto traffic between the park boundary near Hazel Green, through Merced Grove, and down to the Little Nellie Falls area, but I'm not sure when that was. I would guess in the 1950s, but perhaps later.