[SalesForce] How to use apex:actionRegion with required fields and apex:pageBlockSection

I have a custom object with a lookup field to Opportunity. When the opportunity lookup field is changed, I have apex code that gets some data and populates some fields on a portion of the page. So, I'm using apex:actionSupport to call apex when the lookup changes and rerender the block.

The problem is, there's a required field inside the same block that is validating. I realize the solution to this is to use apex:actionRegion and I think I have a good grasp of how that works. However, I can't quite see how to get the placement of the actionRegion correct in this particular instance. Here's one variation of code I've tried:

<apex:pageBlock title="Page block" mode="edit" id="infoblock">

        <apex:pageBlockSection title="Information" columns="1">
            <apex:inputField value="{!myCustomObject.Opportunity__c}">              
                <apex:actionSupport event="onchange" action="{!changeOpportunity}" rerender="infoblock"/>               
            </apex:inputField>

            <!-- THIS IS SENT TO THE SERVER FOR VALIDATION ON OPPORTUNITY LOOKUP CHANGE -->
            <apex:inputField value="{!myCustomObject.MyRequiredField__c}" required="true" /> 

        </apex:pageBlockSection>

        <!-- DOES NOT CONTAIN VALUES ON RERENDER -->
        <apex:actionRegion> 
            <apex:pageBlockSection title="Additional Info" columns="1" id="additionalinfo">         
                <apex:inputField value="{!myCustomObject.Field1__c}" />
                <apex:inputField value="{!myCustomObject.Field3__c}" />
                <apex:inputField value="{!myCustomObject.Field3__c}" />
                <apex:inputField value="{!myCustomObject.Field4__c}" />         
            </apex:pageBlockSection>
        </apex:actionRegion>
    </apex:pageBlock>

Here's another example I tried that did not work (only rerender the additionalinfo block):

<apex:pageBlock title="Page block" mode="edit" id="infoblock">

        <apex:pageBlockSection title="Information" columns="1">
            <apex:inputField value="{!myCustomObject.Opportunity__c}">              
                <apex:actionSupport event="onchange" action="{!changeOpportunity}" rerender="additionalinfo"/>              
            </apex:inputField>

            <apex:inputField value="{!myCustomObject.MyRequiredField__c}" required="true" /> 

        </apex:pageBlockSection>

        <apex:actionRegion> 
            <apex:pageBlockSection title="Additional Info" columns="1" id="additionalinfo">         
                <apex:inputField value="{!myCustomObject.Field1__c}" />
                <apex:inputField value="{!myCustomObject.Field3__c}" />
                <apex:inputField value="{!myCustomObject.Field3__c}" />
                <apex:inputField value="{!myCustomObject.Field4__c}" />         
            </apex:pageBlockSection>
        </apex:actionRegion>
    </apex:pageBlock>

Is there a way to place the actionRegion and rerender the appropriate block so that the additionalinfo block rerenders without validating the required field? NOTE: I can only use the immediate attribute if there's a way to not lose the viewstate values.