I forgot to mention that you also need to implement the configuration SPI:
And of course you need to declare your service classes in the META-INF/services directory:
Hopefully some documentation for how to implement a container will come out of this effort
To provide some more info on how to do a container implementation...
When implementing the ContainerConfiguration SPI, the only thing you really need in there is to return ContainerProfile.CLIENT (this defines the internal configuration of the Arquillian engine). Other things you may want to add to this class are configurable things like connection information etc. Keep it a simple JavaBean and it will be configurable through the Arquillian xml configuration file. At the moment a implementation of this SPI is needed for Arquillian to run at all. (It fails with a NullPointerException if not).
The next thing would be the TestEnrichers SPI, this is for handling in-container EJB/Resource/CDI injection into the TestCase. The default implementations might work fine for WebLogic, and you then only need to provide a implementation of the AuxiliaryArchiveAppender SPI to package up the enricher classes. Look at the JBoss example for how to do that in ShrinkWrap.