Half-hearted REST/HATEOAS reply: this wouldn’t be a problem if you put it in a query parameter (or the body with an appropriate content type to account for URL length restrictions)
These API URLs are completely static and side effect free. We considered using a POST body, but then would lose CDN caching, which would make a difference at xkcd's scale. We also decided that URL length restrictions would not likely matter for real world morse input.
I hadn't considered using a query parameter -- that's a really interesting idea! That seems like it'd have been a more elegant solution than the special cased padding we ended up with.
Especially for static/idem potent resources where URL length isn’t a concern this is exactly the kind of thing query parameters were designed for. If you do revise it to adopt that please credit Roy Fielding :)
This is an explanation of the april 1st XKCD comic checkbox [0], in which you can send an receive messages in morse code via a checkbox. There are many responses and gems hidden inside. For a cheat sheet of what you can do see here: [1]