Spring 3: Inject Default Bean Unless Another Bean Present

I would like to configure Spring via XML such that if a particular bean exists, it will be injected into the target bean. If it does not exist, a different, default bean, will be injected.

For example if I have a file like this

<bean id="carDriver" class="Driver">
  <property name="car" value="SOME EXPRESSION GOES HERE, SEE ATTEMPT BELOW"/>
</bean>

<bead id="defaultCar" class="Car">
  <property name="name" value="Honda Accord"/>
</bean>

And load it, I would like the defaultCar injected into the driver. However, if I also load the following file:

<bean id="customCar" class="FlyingCar">
  <property name="name" value="Rocket Car"/>
  <property name="maxAltitude" value="80000"/>
</bean>

I would want the customCar bean to be used instead of the defaultCar bean. My initial attempt does not work, but I think illustrates what I'm trying to achieve:

<bean id="carDriver" class="Driver">
  <property name="car" value="#{ @customCar eq null ? 'defaultCar' : 'customCar' }"/>
</bean>

I know how to do this with a PropertyPlaceholderConfigurer, but I don't want to have to provide a property file / VM property / environment variable / etc. in addition to the file that contains the custom bean. Thanks!

Update:

Based on the "use a factory bean" comments, I looked into this and came up with the following solution. First, I created a generic factory bean that allows you to specify a default bean name and an override bean name:

public class DefaultOverrideFactoryBean implements FactoryBean, BeanFactoryAware {

    public Object getObject() throws Exception {
        return beanFactory.containsBean(overrideBeanName) ?
               beanFactory.getBean(overrideBeanName)      :
               beanFactory.getBean(d