Affine transformation without imwarp
To find coordinate or index of drew region, first you have to find the coordinate of 4 colored dots, which is easily obtained by using the transformPointsForward function over the intrinsic coordinate of these 4 points in your original image. intrinsic coordinate is the index of your image matrix.

then you have to do some geometric calculations. first, find the length of one of red lines which are the (length of full warped image)/(sin(theta) + cos(theta)), then distance of red line to the nearest colored point is (length of red line)*cos(theta)*sin(theta). theta is the rotation angle. using this distance and the coordinate of 4 colored points you could find the coordinate of your ROI corners.