I don't believe you can set up the system to be selective with use flag depending on some other parameter. You can set the use flag globally in make.conf, or you can set it on a package basis in /etc/portage/package.use/
For example (made up package): create file /etc/portage/package.use/test with the contents:
This will enable the examples use flag for all versions of the test-libs/test package.
Have a look at the portage man page for more information on how to set flags for different versions of a package.
Hope that helps!