Since the ports infrastructure requires that every port have a CATEGORY= that is in the list of valid categories, please add 'local' to VALID_CATEGORIES to allow for local ports within an organization.
You can do this many ways yourself, without patching anything in the ports tree, for example, what I do is in the port Makefile, put: CATEGORY= local VALID_CATEGORIES+= local
I think the best thing is probably to entirely relax the concept of category. I never understood the reasoning behind that concept beside issues with make index probably. adding a local category by default sound reasonable to me until we come with something better.
More proper places are: /etc/make.conf Mk/bsd.port.local.mk I've written a ports_make.conf thing at work too I can should share if interested.