Not sure if you are still trying to do this, or if you have had any success in inducing necroptosis within trophoblast?
Using TLR agonists (such as LPS or Poly I:C) in combination with zVAD induces necroptosis very nicely in macrophage cells. This may also work in trophoblast.
You might also consider hitting the cells with some interferon-beta, as it also can induce necroptosis if combined with caspase inhibition (zVAD).