Hi,
I do not know the details that you are asking for, but, recently, I had been to trade show ELECRAMA in Mumbai,INDIA, and UC-Microsystems is coming up with kits and add-on modules, for training people in MCS51.
One of their Add-on Module is using P89V51RD2 with a direct USB connectivity.
Atleast, this is what I saw during the show. They will obviously will not give away the technical details how it is done.
But talk to them,....if you are luck...who knows!
If you purchase the kit, maybe they will give you those details.