I want to write a web application in which part of the database schema is stable (i.e. will not change after the application has started) and part of the schema is dynamic (i.e. at runtime it is possible for users to create , alter and drop tables).
I was wondering if it is possible to use Hibernate for this. I read something on the createMappings function, but there is so little documentation about it, that I don't know if and how I can use this.

Is it possible to use Hibernate merely to abstract from the underlying database specific code? In that case I could use Hibernate mappings for the stable part of the schema and execute the necessary queries for the dynamic part using Hibernate to figure out how to translate the results coming from Postgres, MySQL, Oracle etc. and their different versions. This would be especially helpful when handling BLOBS.

Hibernate is a good tool with database. As long as you have your mapping file correct, make sure that you do have proper catalog in your database there is no need for you to create tables. Hibernate will automatically create tables for you. Even if you alter your objects (add new properties), Hibernate will automatically updates your tables.

You could look at using hibernate 3's other entity modes, e.g. DOM, so that you can create mappings whilst the application is running, re-create the session factory with the new mappings, and use DOM models to perform the actions you would otherwise do with objects.

See the dynamic models section of the reference manual at:
Dynamic Models