This has apparently been supported in Portage for some time, but I only learned it recently from a gentoo-dev mail: you do not have to write down the expanded USE-flags in package.use anymore (or set them in make.conf)!
For example, if I wanted to set some APACHE2_MODULES and a custom APACHE2_MPM, the standard package.use entry would be something like:
www-servers/apache apache2_modules_proxy apache2_modules_proxy apache2_modules_proxy_http apache2_mpms_event ssl
Not as pretty/convenient as a ‘APACHE2_MODULES=”proxy proxy_http”‘ line in make.conf. Here is the best-of-both-worlds syntax (also supported in Paludis apparently):
www-servers/apache ssl APACHE2_MODULES: proxy proxy_http APACHE2_MPMS: event
Or if you use python 2.7 as your main python interpreter, set 3.4 for libreoffice-5.1 😉
app-office/libreoffice PYTHON_SINGLE_TARGET: python3_4
Have fun cleaning your package.use file