<apex:component access="global">
    <apex:attribute name="record" type="MyObject__c" />
</apex:component>

<messaging:emailTemplate subject="..."
                         recipientType="User"
                         replyTo="..."
                         relatedToType="MyObject__c"
                         language="{!recipient.LanguageLocaleKey}">
    <c:myComponent record="{!relatedTo}" />
</messaging:emailTemplate>

<apex:outputText value="{!relatedTo.SomeField__c}" />