It is not possible to get all the hosts of a specific vCenter folder from a single Data Manager query. You have to get the list of datacenters first and then get the list of hosts from each datacenter.
It is better to make these repeated queries at the java level and only return the list you want to the UI.
You can get all the host objects from the system with a simple query using a Constraint with targetType = "HostSystem" but you would have to filter out the ones from other vCenter servers. See how this chassis sample queries all the hosts from the java later in method getHosts(): samples/chassis-app/chassisRackVSphere-service/src/main/java/com/vmware/samples/chassisRackVSphere/ChassisRackVSphereDataAdapter.java
Another option is to use the vSphere Web Services SDK to traverse the vCenter tree. See the vSphere Management forum for help on those APIs. See this sample plugin which is using that SDK