Hi,
yes jbpm6 will support that strategy, here you can find all the initial supported strategies: https://github.com/droolsjbpm/jbpm/blob/master/jbpm-runtime-manager/src/main/java/org/jbpm/runtime/manager/impl/RuntimeManagerFactoryImpl.java
If I understand your question about the Singleton Strategy, it depends on the use case. If you create a session per process instance it will work fine, I'm not sure why you mention that is not a practical strategy.
Cheers