http://www.zkoss.org/javadoc/6.5.2/zk/org/zkoss/zk/ui/sys/IdGenerator.html says that there is a way to get deterministic IDs to implement this interface in some way.

I say ZK should go with such an implementation and allow just a simple property zk.id.generation.deterministic=true to allow quick start of UI testing basing on component identifiers.