It is possible to use 'in' operator with a SnapshotStore,which retrieves data from Lookback API, as in the example below, where 1111 and 2222 are OIDs of PortfolioItems of type Theme:
Ext.create('Rally.data.lookback.SnapshotStore', {
context: {
workspace: this.context.getWorkspace(),
},
find: '{'+' "_ItemHierarchy":{$in:[1111,2222]},'+
'"_TypeHierarchy":"HierarchicalRequirement"'+'}',
//...........
'in' operator is specifc to LBAPI. In config options of Rally.data.QueryFilter there is a list of valid operators, and it does not include 'in'.