Write your own BeanFactoryPostProcessor?
I think the only solution is to write your own BeanFactoryPostProcessor. The Spring property placeholders work really well, but they aren't designed to be extensible in this way. Fortunately they are pretty simple. You probably need to copy and adapt the code from the visitor (private inner class) in PropertyPlaceholderConfigurer.
If you have a large amount of data in your properties file, you might want to consider Apache commons logging as an alternative (once you are writing your own placeholder configurer, you are not tied to properties files). With commons logging you can have a hierarchy of entries, like in your example, with overrides and defaults built in. On the other hand, if you have something that already works, why fix it?