I don’t know how many people know about per-package environment variables in portage since 2.1.9 or so. (ref: bug 44796) It is a worthwhile enhancement to know about, regardless. Like most people, I have my PORTAGE_TMPDIR on tmpfs to speed up compilation times and reduce I/O usage. My 2G tmpfs mounted on /var/tmp/portage is large enough for almost all packages, even multiple jobs at once, however, not all. Solution:

% cat /etc/portage/package.env
app-office/libreoffice notmpfs.conf
% cat /etc/portage/env/notmpfs.conf
PORTAGE_TMPDIR=“/var/tmp/notmpfs”

(More info available in the portage man page)

Now, when I find my next package that needs notmpfs, it is as easy as: echo "cat-egory/pkg notmpfs.conf" >> /etc/portage/package.env which is much easier than bashrc hacks or something else insane that I have seen. Of course you can extend that to most make.conf settings, hope that helps someone.