Maybe you could get the rotation of the bone that you are attaching the socket to? So something along the lines of, having the attach socket, somewhere along the middle of the thigh, call it ‘thigh_attach_socket’. Then have 2 more sockets, one at each end of the ‘thigh bone’, a start and an end. Call them ‘thigh_line_up_start’ and ’ ‘thigh_line_up_end’.
Then you can attach sphere components in the blueprint, called ‘ThighAttach’, ‘ThighStart’ and ‘ThighEnd’. You could then get the rotation for those sockets and set the rotation for your ‘ThighAttach’ to the rotation vector of the rotation vector between the ThighStart and ThighEnd (FindLookAtRotation()).
Something along those lines?