[SalesForce] Returning an SObject from an InvocableMethod to a Flow

I'm trying to build a bulkified version of Flow's Fast Lookup element so that I can safely trigger bulkified flows from process builder.

I've written an InvocableMethod which allows me to pass in a list of Ids and return a list of SObjects with the appropriate fields queried. These work perfectly with the test class I've written with them, but when invoked from a flow they fail:

public class UserFastLookup {

    @InvocableMethod(label='Retrieve User' description='Returns the User requested, acts as a bulkified version of Fast Lookup')
    public static list<User> getUsers(list<Id> requestIds) {

        //query for user Ids requested
        map<Id,User> recordMap = new map<Id,User>([SELECT Id, Name FROM User WHERE Id IN :requestIds]);

        list<User> userResults = new list<User>();

        //return Users in same order as request to Flow
        for (Id request : requestIds) {
             userResults.add(recordMap.get(request));
        }

        return userResults;
    }
}

I've created a very simple flow, which invokes the Apex InvocableMethod with an Id passed into it, captures the User object output into an SObject variable, and dumps it to the screen (as a simplification to the headless flow using a record update, which exhibits the same errors).

<?xml version="1.0" encoding="UTF-8"?>
<Flow xmlns="http://soap.sforce.com/2006/04/metadata">
    <actionCalls>
        <name>Lookup_User</name>
        <label>Lookup User</label>
        <locationX>196</locationX>
        <locationY>237</locationY>
        <actionName>UserFastLookup</actionName>
        <actionType>apex</actionType>
        <connector>
            <targetReference>Display</targetReference>
        </connector>
        <inputParameters>
            <name>requests</name>
            <value>
                <elementReference>userId</elementReference>
            </value>
        </inputParameters>
        <outputParameters>
            <assignToReference>usr</assignToReference>
            <name>output</name>
        </outputParameters>
    </actionCalls>
    <interviewLabel>User Lookup Test {!$Flow.CurrentDateTime}</interviewLabel>
    <label>User Lookup Test</label>
    <processType>Flow</processType>
    <screens>
        <name>Display</name>
        <label>Display</label>
        <locationX>507</locationX>
        <locationY>242</locationY>
        <allowBack>true</allowBack>
        <allowFinish>true</allowFinish>
        <allowPause>false</allowPause>
        <fields>
            <name>Output</name>
            <fieldText>{!usr.Name}</fieldText>
            <fieldType>DisplayText</fieldType>
        </fields>
    </screens>
    <startElementReference>Lookup_User</startElementReference>
    <variables>
        <name>userId</name>
        <dataType>String</dataType>
        <isCollection>false</isCollection>
        <isInput>true</isInput>
        <isOutput>false</isOutput>
    </variables>
    <variables>
        <name>usr</name>
        <dataType>SObject</dataType>
        <isCollection>false</isCollection>
        <isInput>false</isInput>
        <isOutput>false</isOutput>
        <objectType>User</objectType>
    </variables>
</Flow>

This fails with the following error:

FLOW_ELEMENT_ERROR The flow failed to access the value for usr.Name because it hasn't been set or assigned.|FlowScreen|Display

If I rewrite the code to return a list of strings it works perfectly (and plugs into process builder and works perfectly for bulk operations, as described here).

@InvocableMethod
public static list<String> getUsers(list<Id> requestIds) {}

I've also attempted to define the SObject within an InvocableVariable for the return type, again no luck

@InvocableMethod
public static list<Result> getUsers(list<Id> requestIds) {}

public class Result {
    @InvocableVariable(label='User' description='Retrieved User Object')
    public User usr;
}

Can anyone see where I'm going wrong with this, or has anyone successfully passed SObject from an Apex Invocable Method to a Flow?