We use openwebauth, which is my own creation and grew out of the work we did in Friendica a decade ago to bring Facebook-like interactions to the fediverse. In its latest form it’s just webfinger and http-signatures used to provide web authorisation to a page on another domain. It’s completely separate from whatever message transfer protocol you use.
The issue with wall-to-wall posts is that signatures don’t work, but signatures don’t work in the fediverse anyway so it isn’t a huge loss. The post is signed by the wall owner instead of the author. I think Mastodon will reject these, but I don’t really care. It’s just one of hundreds of Mastodon quirks we have to live with.
Also is there any kind of capability indication that an actor accepts wall posts?
This is a permission and we don’t usually share our personal permission details with strangers. Basically if you visit a page on a remote site and you get a post or comment form (and for some people an upload button) you have permission to use it. The Zot protocol provides access to your own permissions so you can see this ability remotely and react accordingly. I haven’t converted that concept to ActivityPub because it uses a completely different permission implementation which has no direct association or mapping to individual actors.