sharing properties between modules

[EJ Ciramella]

How are people sharing common (truly) properties between two detached
modules.  
 
Example, module A and module B need foo=bar.  It's imperative that both
modules have the same resolution yet they don't share a parent.
 
How do you put foo=bar in one place and see to it that both get it
resolved (without putting it in settings.xml etc).