Getting packed spritesheet data in script?

I can’t seem to find any references to this in the Lua API docs. I’m writing a script where I need access to cel data before I export it (so I can get values like position of one layer relative to another), but also after I generate a packed spritesheet. The end result would be a file describing both. Obviously, I can get the unpacked cel data using the Lua API, but what about the packed export cel data? Is there a way to access this before/after export with the Lua API, or a way to get/set JSON values before the file is generated?