There are at least two ways to do this with IMWARP. One is to use a composite geometric transformation in which you pre and post multiply your intended transformation to shift the image to the origin (0,0), apply your affine operation, and then shift the image back to its original center.
A second, and I think easier way to do it, is to put your image into a non-default coordinate system where it is defined as being centered at the origin and then just apply your intended transformation. This approach is shown below.
A = imread('pout.tif');
Rin = imref2d(size(A))
Rin.XWorldLimits = Rin.XWorldLimits-mean(Rin.XWorldLimits);
Rin.YWorldLimits = Rin.YWorldLimits-mean(Rin.YWorldLimits);
out = imwarp(A,Rin,tform);