@Brian_Ghilliotti I got this back from a resident of Barre MA:
Rum Rock is in the wilderness and it does not have an address. It is located within a very large Massachusetts Audubon Sanctuary that stretches from Petersham into Barre (Rutland Brook Sanctuary). Rum Rock is reachable only by hiking on woods trails.
The entrance closest to the “Rum Rock Loop Trail” is located off Pat Connor Road in Petersham, which is an extension of Butterworth Road in Barre. Butterworth Road is a narrow dirt road off of Mass. Route 122 in Barre, almost at the Petersham town line. It is about .7 miles up Butterworth Road, onto Pat Connor Road. There is a tiny spot to park, then you start hiking. There is a well marked hiking trail, Rum Rock Loop Trail, which is a rough woods path that is quite long. This is a densely wooded area of swamps and beaver ponds, and many cliffs that contain granite glacial erratics similar Rum Rock.
I hiked this area for three hours this afternoon and I cannot tell you definitely that I found Rum Rock, because it is not marked and there are several formations very much like it. Massachusetts Audubon created the Rum Rock Loop Trail about 2008 and hosts an annual walk to Rum Rock, usually in October, but they do not publish the location of the trail or the rock.
A hunting friend who is familiar with this area estimates the geographic coordinates as 42.45282, -72.15139. This is an estimate only, not done at location.