Roger, This might help...from the book about the X10.
"If image size is set to Large, the camera cannot use the special features of the EXR sensor, it must use all 12 mp for resolution. If image size is set to M or S, then the EXR technology is available..."
"In M or S, then the camera is able to devote at least half of its sensor capacity to DR processing. In that case, if you set DR to 400% then you will see you are able to set ISO to any value you want , including 100, because the camera is now using the EXR technology to achieve DR..."
If the image was set to L...in EXR if the DR was set to 400% he would have to use at least an ISO of 400. I don;t know if he had data on image size.
Auto EXR I believe will use ISO 100 but it will move DR to 100%.