The man-page make.conf(5) as well as several places in the Wiki and the online documentation state that I can set BUILD_PREFIX to some directory to override the default "${PORTAGE_TMPDIR}/portage". But that doesn't work, such a setting is simply ignored, regardless of whether I set it in /etc/make.conf or in the environment. All I can do is change PORTAGE_TMPDIR, but that changes more than I want. I guess the reason is that the value is just hardcoded in line 3919 of /usr/lib64/portage/pym/portage.py. So either fix it there or change all the documentaion, please. I'm currently using sys-apps/portage-2.1.4.4 on amd64.
Thanks, I've removed it from the man page for the time being since it's not intended to be configurable.
This is released in 2.2_rc7.