Micromet was developing a set of bi-functional antibodies that pretty much accomplished this same thing. That is directing the immune system to tumors. One side of the antibody would bind the tumor, while the other bound the T-cells. They were bought out by Merck a year or two ago and I believe the therapies are in clinical trials. You can read about the antibodies here: http://cancerres.aacrjournals.org/content/69/12/4941.full.pd...

The added benefit here is that the antibodies can be produced en masse and delivered as a drug rather than reprogramming the person's T-cells.