I can understand your desire to modify parts of an eqr image held in photoshop. Bernard Custard especially, others too, make similar requests. It seems the key operation is converting from eqr to rectilinear and back again, that PS does not do. Everything else should be easy for a PS expert.
Is this the right idea? You make a selection in a 360x180 equirectangular, and mark a point as the optical center. The plugin converts the selection to rectilinear, with that optical center, and gives it back to PS as a new image. Some data are saved so that later the extracted image can be converted back to eqr, at any position on the panosphere (including rotated), and used as an overlay on the orginal eqr.
I could easily make Panini-Pro move a selection around on the panosphere and paste it in a new place. But of course you guys need to edit the selection, then mask and blend it back just so; for that I agree it has to be in PS.
I am no kind of professional PS plugin developer. Though I have built a few simple ones, what we are talking about here is not an elementary PS filter. So if anyone knows someone who is a pro PS filter developer, and might be interested to work with me on this (for eventual or even immediate money) please do help me get in touch with them.
As for going straight from PS into the full Panini-Pro, I don't think that is important enough to justify developing a special version of Panini-Pro that can be launched under PS. Just save the image from PS, load it in P-P, cut a view and load that into PS. This could of course be "automated" the way exchanges of images between PS and LR or Bridge are, but that is just head-candy.
I am seriously going to build a new Panini version for less than top-end
hardware. I need a more universal engine to support these PS plugins anyhow,
and the open source Panini is long overdue for an upgrade. It will
definitely run on a Thinkpad T60, which I consider about the bottom end
for photo work.