Albeit a little late the answer to your question is: yes, Oracle WebLogic Server can be clustered without doing an OS cluster. You can create WebLogic Managed Servers on different machines and cluster them horizontally and use a proxy server for load balancing or alternatively you could create multiple WebLogic Managed Servers on the same machine having different ports and cluster them vertically and use an Apache Web Server to do your load balancing or even a hardware load balancer if you like. There should be lots of docs online to help with this.

Happy clustering,