A suggestion: Why not make the original alt text into alt text here as well? It often acts as a punchline, and having it in the post text kind of blunts the effect.
Is the weird (ab)use of content warning (itself an abuse of subject) not giving you a similar effect (that you see the original alt text only after performing an explicit action)?
That might work if I didn't have my client set to ignore content warnings -- I live life on the edge. 🙂 And as you say, this is pretty clearly a misuse of the CW feature. Publishing the original alt text as alt text seems a lot simpler and more faithful to the original comic.
Would setting CW (vel subject) to "spoiler" also be a misuse of it?
Putting original alt text would be confusing for people who don't see images and are unfamiliar with xkcd; it's after all _not_ a good description of the image.