I found a blog entitled SAGA and GRASS running in ArcGIS through SEXTANTE which says:
This time, I am showing how SAGA and SEXTANTE algorithms can be
executed just like native SEXTANTE algorithms. This is completely true
for SAGA ones, but I'm having a bit of trouble with GRASS ones,
specially vector ones. This is not a GRASS or SEXTANTE problem, but
just a problem caused by the strange behaviour of Java when I call the
application (SAGA or GRASS) from ArcGIS, since it freezes and I
haven't been able to solve it.
I have not been able to uncover any other reports of anyone succeeding at doing what you hope to.