Steve,
Rules are indeed bound to a deployment. This allows isolation of rules/processes in their different versions, keeping the right ones together. If you want to have a "shared" knowledge-base, I suggest you create a custom JavaDelegate. Instead of using a knowledge-base based on deploymentId, you locate it using your own mechanism, exposing the "global" rules.
Take a look at the implementation of org.activiti.engine.impl.bpmn.behavior.BusinessRuleTaskActivityBehavior to make rules work nicely together with the activiti-context.