Hi versionz,
It’s not yet clear for me if your link is about simply creating custom fields for attachments in general (which ACF can do finger in the nose), or custom fields for —post attachments— (as looks like you are talking about), meaning they would —only— apply the gallery images of my actual post and would then answer my need.
Sadly, even if it was the case, the amount of effort to put on this (looking at the link you provided) is way too much for the need. I’m actually able to capture events directly from ACF with a minimum of effort (see above with John examples, and some js). The only thing I would love to be more clean, is to capture the “Add size” button event.
