[gentoo-amd64] Change Install Prefix

[Frank Peters]

Is there a way, aside from writing a custom ebuild script, of changing
the install prefix for a package?  What I want to do is install some
packages under /usr/local or /opt rather than the default of /usr.
Unless I am mistaken, there does not seem to be a way of doing this
with either portage variables or with portage profiles.

Using the ROOT variable is not feasible because the resulting
emerge will create an independent tree of packages.  I just want
to shift some packages out of /usr without losing any dependency
or linking information.

Linux is a flexible system in that it will allow libraries, include
files, and configure information to be stored in a variety of places
without losing any interaction.  Modification of PATH, PKG_CONFIG_PATH,
CPPFLAGS, LDFLAGS, and other variables as well as the /etc/ld.so.conf
file will keep a system integrated no matter where things are stored.
Yet, unless I am missing something, Gentoo does not seem to embody this
same flexibility, at least not in a simple manner.  All packages have
to go under /usr only.

Is this actually the case or is there a way to change the installation
prefix?

Frank Peters