There is no ready-made solution for this AFAIK. This article tells how to backup and restore the whole wiki or just a single wiki (or blog?), but the output created by "wikiadmin export" seems to be some binary dump of the database which can be fed only to "wikiadmin import".
The Lion Server Blog/Wiki itself is kept in a Postgres database named "collab", which you can access from the server's command line:
sudo -u _postgres psql collab. The table structure seems quite complex, though - I have a test wiki with 2 documents and it still has 67 tables. You might want to try some graphical frontend for Postgres
to look through the tables and export those needed.