I think this strange "default" spine pose is the way Michael4 model is created by DAZ3D. I did not realize there was a problem until you pointed out to it.
The workaround is to edit the first frame of BVH file with Michael4 model to make spine look more natural in default pose. You can use DAZ3D or BVHacker (
http://www.bvhacker.com ) to edit the pose but this may end up to be a trial-and-error process. When you use "Import target character" menu item, iPiStudio treats the first frame of imported BVH as T-pose. So by editing the first frame you can adjust motion transfer.
We will talk to DAZ about this problem but coming up with a more graceful solution may take much time (as usual with this kind of integration issues).