The Wifi backpack already exists for the older models.
The Hero3 has Wifi built in. And yes, the "live" (1-2 second lag) display on
a smart phone would be useful for initial aiming. That's usually the most painful bit.
To give you an idea of FoV, my original Weldon DS Experience vid was shot using
1080p at 127 degrees FoV on the Hero HD. That's now called "medium wide" setting
on the Hero2 and Hero3.
I shot a frontside acro vid this summer using Hero2's 1080p narrow FoV (90 degrees).
Here's the two vids for comparison
1080p 127degrees FoV
1080p 90 degrees FoV
My preference is for the latter but the aiming can be a real pain. I shot
most of that footage twice, because I had it mis-aimed high the first time.