At this point, because of the very helpful work done by Baptiste, it is possible to federate with Plume now. There are still some minor ActivityPub violations, but it is now in a pretty good state.
People are probably wondering: how does a Plume post look in Pleroma. As Mastodon users may know, Plume posts are converted to a simple representation of the activity, since Mastodon does not internally use ActivityPub. But as Pleroma does use ActivityPub internally, we can do better.
This is what a Plume post looks like in Pleroma frontend:

And this is what a Plume post looks like in Mastodon frontend, hosted on Pleroma:

To compare, Mastodon backend renders Plume posts like this:

This functionality will be enabled in Pleroma mainline early next week.
Comments
and, this is what a real Plume post looks like in Pleroma FE.
image.png
@kaniini this is so exciting. #ActivityPub
@kaniini @kaniini @0x1C3B00DA @wakest @kaniini @federationbot
Why is this happening?
@kaniini @kaniini @0x1C3B00DA @wakest @kaniini @federationbot
Nvm, for a second I though the pleroma one showed the full article and mastodon didn't. Carry on.
Mastodon *doesn't* show the full article. Pleroma does.
@federationbot @kaniini @wakest @0x1C3B00DA @kaniini @kaniini oh, okay. Do you know why?
yes. Mastodon does not understand Article objects in a useful way so it just converts it to a Note.
@kaniini @0x1C3B00DA @wakest @kaniini @federationbot @kaniini Article objects? Is that something that's new with Plume?