you could always logically separate them in different layer2 domains. That will allow you to separate which APs should broadcast which SSIDs.
If I'm correct, it will be possible in the next release of Unleashed to to create separate AP or WLAN groups in Unleashed
What specific use case are you looking for?
You want to have 1 cluster but be able to selectively choose which APs will broadcast which SSIDs? Sort of like
Zone A -> broadcast SSID 1 on all APs.
Zone B -> broadcast SSID 1 and 2 on AP 1 and 2 but not 3 and 4?