This appears to already exist https://chromium-review.googlesource.com/admin/repos/chromiumos/third_party/virglrenderer,branches

It was requested in https://bugs.chromium.org/p/chromium/issues/detail?id=852942 

I guess this just needs to be added to the manifest? If we want this in the regular checkouts it needs to be in both the internal and public manifests.

We decided back then to not have a source tree but instead just rely on pulling forward the hash and having patches if we need them.

Fair enough, I guess for testing complex changes one can just squash and do an epatch.

You can also do something like:
ebuild-$BOARD ~/trunk/src/third_party/portage-stable/media-libs/virglrenderer/virglrenderer-9999.ebuild unpack
cd /build/$BOARD/tmp/portage/media-libs/virglrenderer-9999/work/virglrenderer-9999
git init
git add .
git commit -m 'initial checkin'
git remote add origin git://git.freedesktop.org/git/virglrenderer
git remote update
git checkout -B master origin/master
ebuild-$BOARD ~/trunk/src/third_party/portage-stable/media-libs/virglrenderer/virglrenderer-9999.ebuild configure
git commit -am 'configured'
git branch master-configured

# repeat the following steps as much as needed
rm -f ../../.compiled; ebuild-$BOARD ~/trunk/src/third_party/portage-stable/media-libs/virglrenderer/virglrenderer-9999.ebuild compile
scp src/.libs/libvirglrenderer.so.0 root@$DUT:/usr/lib64

Be careful because once you do an ebuild-$BOARD XYZ install or merge, it will nuke the working directory unless you run those commands with FEATURES=noclean.  All the git stuff I listed above is optional, but lets you get diffs of your changes or apply them relatively easily.