I'm not sure where we should keep these variables (profiles?), but having them hard coded in portage is certainly bad.
Hmm, portage_const.INCREMENTALS is another one that we may want to define via the profiles.
In svn r4452 I've added support for SPACE_SEPARATED and COLON_SEPARATED in /etc/env.d/ so that hard coded values are not required.
This has been released in 2.1.2_pre1.