Well to aim to her feet all you have to do is keep the cursor in the lower part of the screen for a few seconds, what I mean is that the remote also knows when the cursor leaves the screen completely and when it enters it again, so in that scenario the cannon would dissapear while it still is looking down, afterall if you are not pointing at the screen you aren't attacking anything, just staring low.
Thats another thing, it would be another nice detail to see more than just the arms, but the legs as well, I remember a poster here (I forgot his name) who suggested this, is not necessary to see it all the time, one of the details of the prime games is that Samus has limited movement, and with the way the armor is it made a lot of sense, it becomes difficult to aim completely up or completly down because of the suit, so the legs would be seen only after landing from a dramatic jump or something like that.