Thanks for your feedback, please see my comments inline.
a) Firstly, all reports crashed due to lack of jdbc driver for hsqldb, which I assumed was present in tomcat lib directory. But obviously that was not enough. After some investigation the problem was caused by BIRT that assumed jdbc driver to be present in completely different location (CATALINA_HOME/birt/ReportEngine/plugins/org.eclipse.birt.report.data.oda.jdbc_2.3.2.r232_v20090212/drivers). By placing hsqldb driver there two of the reports started to work properly.
b) But another problem pop up with other reports. It tried to load MySQL driver instead of hsqldb.
The report templates use the JDBC driver at design time, within Eclipse but should refer to a jbpm datasource instead when used within tomcat or jboss. So we are actually talking about two different problems here:
Within tomcat it should be using the JDBC connection, but instead reference a datasource. Otherwise, you depend on the JDBC connection details encoded in the report templates (see description below). In that case you cannot switch databases without modifying the report templates in eclipse.
This requires manual datasource configuration for tomcat, because tomcat uses readonly JNDI.
Whatever JDBC connection you use at design time (mysql & hsql) will be encoded in the BIRT templates. Although referencing the datasource, BIRT for some reason still tries to initialize the JDBC connection. This leads to an error being logged, but the report generation still works.
Let me know if that helps.
Thanks Heiko for explanations. I managed to find out what was going on.
At the moment I ran into problems with PostgreSql wile running some reports. BIRT complains about missing data sets. I will try to post exact error messages soon.