I made changes to the cartridge to implement this. It's hard to make a patch since these changes concern some templates that are already concerned by the patch to separate daos and transformers, so such a patch would work only if you have applied patch for dao / transformers before.

Anyway, you don't need a lot of changes to make it work.

In EJB3EntityFacadeLogicImpl class, you have to change implementation for handleIsDaoImplementationRequired method :

/**
 * @see org.andromda.cartridges.ejb3.metafacades.EJB3EntityFacadeLogic#handleIsDaoImplementationRequired()
 */
protected boolean handleIsDaoImplementationRequired()
{
    return !this.getDaoBusinessOperations().isEmpty();
}

In cartridge.xml.vsl template you have to remove these lines :

<template
    path="templates/ejb3/DaoImpl.vsl"
    outputPattern="$generatedFile"
    outlet="daos"
    overwrite="true">
    <modelElements variable="entity">
        <modelElement>
            <type name="org.andromda.cartridges.ejb3.metafacades.EJB3EntityFacade">
                <property name="daoImplementationRequired">false</property>
                <property name="embeddableSuperclass">false</property>
            </type>
        </modelElement>
    </modelElements>
</template>

In ejb-jar.xml.vsl template, you have to change the way dao session beans are defined :

<session>
    <description>
        <![CDATA[
        $entity.getDocumentation("", 64, false)
        ]]>
    </description>
    <ejb-name>${entity.daoName}</ejb-name>
    <local>${entity.fullyQualifiedDaoName}</local>
    <ejb-class>#if ($entity.daoImplementationRequired)${entity.fullyQualifiedDaoImplementationName}#else${entity.fullyQualifiedDaoBaseName}#end</ejb-class>
    <session-type>Stateless</session-type>
    <transaction-type>Container</transaction-type>
</session>

And finally in DaoBase.vsl template you have to change the class definition line so that the class is abstract only if there is a Dao implementation :

public#if ($entity.daoImplementationRequired) abstract#end class $entity.daoBaseName