Equirectangular to Gnomonic and back - for VR filmmaking

[cly3d]

Hi All,
I never thought I'd be speaking to Cartographers about film-making, but I suspect this forum is going to get flooded in the next few months, and Cartographers are going to be in vogue once the VR filmmaking community discover you.

My search for "compositing" regular elements over an equirectangular image with the correct remap/distortion is what brings me here.
What I would like to do is;

• I have a full "dome" image from a 360 lens camera (ricoh) The camera stitches the image internally into an equirectangular image for panoramic viewing
• I'd like to straighten out this equirectangular image into a normal projection (Gnomonic?) in software such as Photoshop or aftereffects
• Once straightened out, I'd like to add regular elements such as Text, signs, people etc..
• Finally, re-warp the image back to it's original equirectangular projection.

I've attached an image example.

I've downloaded G-projection, but when I choose gnomonic mapping It doesn't show the "whole" image, just a section of it. Could it be because the image is 3584 pixels wide?
Secondly, would G-projection then be able to wrap such a gnomonic projection image back to equirectangular?

Any help much appreciated.
Kind Regards.