The next -- and Enterprise -- version of the BlogMatrix Platform will support the Metaweblog API; that is, you will be able to post to BlogMatrix using posting tools such as Windows Live Writer (my favorite, though don't let it screw up your IE settings) or w.Bloggar.
The official "endpoint" for Metaweblog XML/RPC calls will be: http://userid.blogmatrix.com/:admin/xmlrpc/metaweblog/.
At some point we'd like to support the Atom API, Google GData-style, but library support on Python at this time is lacking.