contolini mentioned working around this issue by posting large messages to anonymous GitHub gists.
What if the bot API automatically uploaded large messages as if they were text file attachments?
The message portion of the post could contain an automatically-generated snippet preview of the text attachment contents.
And clients could be improved so that when a user clicks on a text file attachment, it displays the text inline inside a pane with a scrollbar, so that the user doesn't have to download the text file locally in order to read the full message.