I am using MySql 4.0.23, and I want to use

<property name="area" formula="( SELECT (l.rows * l.cols) FROM landscape l where l.id = id ) " type="int"/>

but this results in SQL

select ..., ( SELECT (l.rows * l.cols) FROM landscape l where l.id = this_.id ) as formula0_0_, ... from landscape this_ where ...

but MySql 4.0 doesn't support subqueries. So I get:

You have an error in your SQL syntax. Check the manual that corresponds to your MySQL server version for the right syntax ...

Is there a workaround?

There is nice workaround:
Postgres : http://www.postgresql.org/

MySQL is well known for non-standard SQL and lack of support for SQL standards.