You may not be aware, but the on-screen rectangle has left, right, top, and bottom position handles that only change the size in that direction. However, those handles are disabled when the “Size mode” is “Fill” (which also happens to be the default). This effectively provides the H/V constraint you are requesting.
Also, when rotating, the rotation snaps to 0, 90, 180, and 270 degrees.
I have updated the documentation to explain these features.