It's easy to show that the WTS doctrine is that Jesus only is mediator for the 140,000. They are open about that and base it on the interpretation of the new covenant. The question was if Jesus died for the GC. The mediation of the New Covenant is not the sole consideration in the answer to this question. The WTS literature does often differentiate between the GC and 144,000 when discussing salvation but is clear that salvation is offered to the GC through their exercising of faith in Jesus' sacrifice. They also state he died for all.
So, it can be argued that JW publications show Jesus strictly only really died for the 140,000 since he only mediates for them on the NC but I think the position on the NC does not preclude his death having the power to redeem the of all mankind.
I will await Terry's POV on this when he provides his answers...
[EDIT] - just to add that of course this apparent contradiction is very well described in the jwfacts.com article. I think that even one of the GB would hold the line that JWs teach Jesus died for all even though he only mediates the NC for the 140,000