At the preseason tryouts for the U.S. Boomerang Team in Greenfield, Mass., one figure stands out among the hacky sack players and pizza delivery guys gathering in the soccer fields, limbering up their throwing arms. Eric Darnell, a soft-spoken 62-year-old Quaker and backyard inventor from South Stafford, Vt., has brought 60 of his latest handmade boomerang prototypes, along with several notebooks,