@voylinux I have pushed a branch which seems to be working with at least the simplest case of AtomPub I can think of:
http://q.zash.se/a1de4e6e8444.txt

Save that file as 'notice.atom' and then:
curl -s --data-binary @notice.atom -u username $atomsvc

$atomsvc is in !GNUsocial's case the same URL as the user's Atom feed, which can be found out through the generic discovery procedure.

I'll try to create some up-to-date documentation with curl examples etc. :)