The list of clusters can be obtained even now via REST APIs.
Ex: http://localhost/rest/workflows/3611b71d-dd11-4867-847a-f8599ccc60b9
<workflow
xmlns:atom="http://www.w3.org/2005/Atom" uuid="3611b71d-dd11-4867-847a-f8599ccc60b9">
<name>Add or expand performance storage class to Infinite Volume</name>
.............................................................................................................
<userInputList>
<userInput>
<name>Cluster</name>
<description>Cluster hosting the Storage Virtual Machine with infinite volume. Must be running Clustered Data ONTAP 8.2 or later.</description>
<type>Query</type>
<allowedValues>
<value>smai-2240-126-38-40</value>
<value>wfa-sim-cluster-sn-1</value>
<value>wfa-sim-cluster-sn-2</value>
</allowedValues>
..........................................................................
In an upcoming release Oct-Nov this year, thru REST it will be possible to get dependent user input list(List of Vserver for selected Cluster)