[PKGNG] How to maintain several packages of the same port?

I hope I can adequately explain what I want. Here goes: for a port that has options, I'd like to have a few different packages in a PKGNG repository, each package containing the same port but with a different set of options.

I can use MASTERDIR and SLAVEDIRS to create slave ports, each with their own set of options, but I can't build/update one without first having to deinstall the other, because they install files into the same place and hence conflict with each other. Moreover, portmaster --clean-packages neatly removes the version that just had to be deinstalled in order to be able to build the other.

Any suggestions how I can conveniently keep someport packages someport-foo and someport-bar in the same PKGNG repository?
You can't do that if you're building the ports on the same host where they get installed. What you need is a package builder that uses a separate jail to build the packages separately from the host system and from each other.

Edit: Yes, staging may help as well since the packages are created from the staging area, not from the files/directories that are already installed.
Thanks, guys. I saw a few threads about staging pass by on freebsd-ports@. I haven't read those yet and kept them for future reference, but now I have an incentive to go read them.