1. A system comprising: a processor; at least one computing storage device implementing: a data integration subsystem configured to maintain a baseline data structure representing a base set of data relationships, said data integration subsystem being further configured to maintain a mapping of said baseline data structure to local data maintained by a plurality of local data subsystems; and a portal subsystem configured to provide a first customizable data structure associated with said baseline data structure for access by an external party, fix said baseline data structure to be unchangeable by said external party through said portal subsystem, create a copy of at least a subset of said baseline data structure, and provide a second customizable data structure associated with said copy of said baseline data structure for access by said external party; wherein said first customizable data structure and said second customizable data structure are independently customizable to represent different custom sets of data relationships, wherein: said baseline data structure comprises a plurality of baseline data records and one or more fixed-type map records that represent one or more relationships between said baseline data records, said fixed-type map records indicating that said one or more relationships between said baseline data records are unchangeable by said external party within said baseline data structure; and said first customizable data structure associated with said baseline data structure comprises a copy of said plurality of baseline data records and one or more customizable-type map records that represent one or more relationships between said baseline data records, said customizable-type map records indicating that said one or more relationships between said baseline data records are changeable for customization by said external party within said first customizable data structure.