To directly do what you want requires

(a) extracting the orig->acpc transform matrix from VolA+acpc.HEAD -- call this transform A

(b) extracting the transformation matrix that registers VolB+orig with VolA+orig from VolB_coreg+orig.HEAD -- call this transform B

(c) Then transform C = A*B takes coordinates from VolB+orig space to VolA+acpc space; once C is computed explicitly, it could be applied using program 3dWarp

The only tricky part to the steps is that you have to extract and multiply the matrices more-or-less manually, since their isn't really a separate program for this purpose. Hmmm -- let me think some more.

An alternative would be to try to use 3dWarpDrive -affine_general to directly register VolB+orig to VolA+acpc -- the problem with this approach is that you would first have to resample VolB+orig to be on the same grid as VolA+acpc for 3dWarpDrive to work at all.