Hi Elizabeth.
There should be a matrix which is saved by the program for the subject to standard space transform. If assume you are using art2, since the registration is nonlinear. The warping matrix will be in nifti format.  I suppose you could use an arithmetic program (like afni 3dcalc or fslmaths) and multiply nifti warping matrix by -1 to get the inverse transform, standard to subject space.
I am sure, also, that Dr Babak will soon reply with his input.
Hope this is helpful.
Christine