This is really awesome !
The Use Case :
You want to just try out a patch for something already in portage.
Instead of creating another ebuild in an overlay you can put your patch in :
For example :
mkdir -p /etc/portage/patches/sys-libs/glibc-2.13-r3/ wget -O /etc/portage/patches/sys-libs/glibc-2.13-r3/fix1.patch "https://bugs.gentoo.org/attachment.cgi?id=279269" emerge -1 glibc
To check if your patch applied you should see the following just after the standard batch of patches:
* Applying user patches from /etc/portage/patches/sys-libs/glibc-2.13-r3 ... * fix1.patch ... [ ok ] * Done with patching