You can do this with the
--prefix argument to
emerge --prefix=/usr/local somepackage
This is equivalent to setting the
EPREFIX environment variable prior to calling
emerge. If you start doing this you'll want to make sure
/usr/local/bin is in your
Aside from that there shouldn't be any problems and from portages point of view there isn't a difference. You also won't have any differences in administration. Unmerging packages will still work the same.
There is one caveat to this -- dependencies brought in from the package you are emerging will also be installed into the new prefix. If you really only want the packages you specifically emerged to be in /usr/local, you'll need to keep this in mind.
If your goal is just to keep track of what packages you've specifically requested to be installed, portage records this in your world file