I sometimes want to put long source blocks in a <details><summary>...</summary>...</details> block when I export to HTML, so that they're tucked away in a collapsible block. I tried using https://github.com/alhassy/org-special-block-extras to define my own #+begin_my_details "summary text" ... #+end_my_details block, but source blocks inside my_details doesn't get fontlocked properly while in the Org file. I wanted to add a :summary attribute to the regular src blocks, and to change the HTML export to wrap the code in details if the summary was specified.

(setq org-babel-exp-code-template "#+begin_src %lang%switches%flags :summary %summary\n%body\n#+end_src")
(defun my-org-html-src-block (src-block _contents info)
  (let* ((result (org-html-src-block src-block _contents info))
         (block-info
          (org-with-point-at (org-element-property :begin src-block)
            (org-babel-get-src-block-info)))         
         (summary (assoc-default :summary (elt block-info 2))))
    (if (member summary '("%summary" ""))
        result
      (format "<details><summary>%s</summary>%s</details>"
              summary
              result))))
(with-eval-after-load 'ox-html
  (map-put! 
   (org-export-backend-transcoders (org-export-get-backend 'html))
   'src-block 'my-org-html-src-block))

So now I can use it by specifying blocks like this:

#+begin_src emacs-lisp :summary "Code for adding a :summary argument and using it during export"
;; code goes here
#+end_src

It took me a bit of digging around to figure this out. When I added the :summary attribute, org-babel-get-src-block-info found it when I was in the Org file, but by the time my-org-html-src-block was called, the block had been replaced with a copy that didn't have the header argument. I dug around using edebug's d command for displaying the backtrace, stepping through various functions. I found out that in the process for exporting source code blocks, org-babel-exp-code replaces the source block with the value of org-babel-exp-code-template, substituting certain values. Adding the summary flag to that and retrieving the summary information using org-babel-get-src-block-info worked. I originally used advice-add to override org-html-src-block, but I think I'll try replacing the transcoder.

Adding custom header arguments could be useful for different export-related tweaks (someone wanted to create an argument for highlighting certain lines but hadn't figured it out in that thread). If there's a more elegant way to do this, I'd love to find out!