@byron @debacle @0 I've not implemented this XEP myself, so I can't tell for sure, but I don't see any reason why you couldn't have a PEP service for that. XMPP clients usually don't do PEP/PubSub themselves (because the server does), so I guess that the clients you have tried just don't do that.
On the other hand, if you are serverless and the the flux is already encrypted, you are already end-2-end encrypted (but it seems that encryption is optional with this XEP).
@Goffi @byron @debacle @0 In XMPP encryption/authentication is always a policy decision, not protocol. So up to you to enforce TLS by not exposing any features except starttls on cleartext stream.
In general Collabora invested quite a lot of effort into that for OLPC (http://wiki.laptop.org/go/Activity_sharing, http://wiki.laptop.org/go/Telepathy_Salut) but it's abandoned since 2010 either.
