Yes, someone asked me to add a tag so they can filter it out, and I tried that over from the web interface.
For the first post (first book only) there was no issue. Edit loaded, I could add the tag, press Save and done. Note: this was on a PC using Chrome
For the second post (all three books) it worked the same way as the API. I could load up the page and edit it, but when pressing save I'd get an error message. However the edit did go through actually and the tag is now there.
Haven't tried to edit the text itself, but don't think it would make a difference - if your browser can load it'd just work.
(On mobile on a low/mid-range Android the App could open up the post without problems, but haven't tried editing. Using a mobile browser - Firefox in my case - the page could load but it was super slow, and buggy, and it also did crash once)
Also as a public service announcement: if you want to filter the post out from your dash the tag is #full-lotr-test
Also if anyone is interested I loaded up the entire LOTR trilogy in TXT file, then wrote a code that split it out into NPF (Neue Post Format, you know the new PDF or whatever) blocks of at most 4000 characters, then simply called the Create Post API with the resulting NPF block. I believe there's around 900 blocks in the second post above, each block containing somewhere between 3000-4000 characters, so close to the theoretical limit