After some testing it looks like the VR on a Nikon at least will work now both when I hold the shutter button and when I hold the focus button and as far as I can tell there’s no harm in half-pressing the shutter button when using rear-focus so that shouldn’t be a problem at all.

I need to go play with rear focus tomorrow and see how I feel about it!

Hey Chris, I am 99.99999% sure that that it how things are with Canon though I am not sure if IS comes back on with the press of the shutter button or if it stays on for a while after you rear focus…. If I remember, I will try listening the next time I am out in the field–that likely in Japan in Winter…. In addition, I would not be making sharp stuff at 1200mm at shutter speeds of 1/60 sec. and 1/80 sec. if IS were not engaged…. artie

My understanding is that the IS is independent of the focus. Try it to see. Use rear focus to get your subject in focus, release it, move the “frame” to achieve desired composition and press the shutter button.