Scouring the docs a little more, I found out that I can do this by providing maven dependencies in the container configuration, like so:
<configuration>
<container>
<containerId>glassfish3x</containerId>
<artifactInstaller>
<groupId>org.glassfish.main.distributions</groupId>
<artifactId>glassfish</artifactId>
<version>${glassfish.version}</version>
</artifactInstaller>
<dependencies>
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
</dependency>
</dependencies>
</container>
<configuration>
<properties>
<cargo.datasource.datasource.mysql>
cargo.datasource.jndi=jdbc/MysqlPool|
cargo.datasource.driver=com.mysql.jdbc.Driver|
cargo.datasource.url=jdbc:mysql://localhost/databasename|
cargo.datasource.transactionsupport=LOCAL_TRANSACTION|
cargo.datasource.username=username|
cargo.datasource.password=password
</cargo.datasource.datasource.mysql>
</properties>
</configuration>
</configuration>
Take note of the mysql dependency. It needs to reference a dependency already defined in my project (which, for my project has a scope of "provided"). This works as expected. :)