Best I can offer is to Reverse engineer the Discourse API and see if the UX is pulling down the information that you require.
I didn’t see in a quick look, so the next thing I’d do is look in the source of the composer.
And a quick look at max_post_length I don’t see it in a serializer, so I think that the Discourse client probably just posts it to see if it’s too long. And then I tested that, and indeed, the client doesn’t know the max length:
