The original images have EXIF/IPTC (as I'm sure you are well aware) and I update the EXIF/IPTC description field (one time) once when I review the images at import time. From that point on, the metadata (time, location, description) for my purposes is finished. Of course I can always go back and revise the description if needed (its rare).
Today I can 'export' my images and automatically append and metadata from any EXIF/IPTC. The resulting JPG image files are 'ready for publication' however the caption (which is automatically added) is now part of the image data (this process if just like adding a border) to a photo.
This is not ideal, because (as you suggest) we can't modify a (now visible) caption, its image data! This is where I was hoping APu could help (i.e. read the EXIF/IPTC from the file and insert a caption), but it seems APu has no capability to read the EXIF and auto caption an image 'container' on page.
Frankly, I'm very surprised this basic capability is missing from APu.
Onward!