For automatic upgrade purposes, it would be useful to be able to mark GARstow packages as being "pure libraries": things that are not of use to the user except in conjunction with other packages. Such library packages could then be removed automatically if nothing else depends on them.
For many existing packages, this would require splitting them into "library" and "tools" packages (as Debian already does); it might be possible to automate this to some degree by having one port install two packages. It would also be necessary to have some mechanism whereby the user can say that they're using a library for something that isn't GARstow-managed, so it shouldn't be removed.