Yale has something similiar to what you ask about - they call it an "Adjustable Sling" Buy pre-done or see the splicing instructions - be sure to read the fine print about derating the strength of the rope for no taper and any angle in the legs.
http://www.yalecordage.com/html/pdf/...e/low/Pg30.pdf
http://www.yalecordage.com/html/pdf/yale_backsplice.pdf
mb