There are a number of discussion papers available from MARBI . You may find them interesting to note.
Recording geographic coordinates in the MARC 21 Authority Format
Addition of coded value to 008 for content alerts in the MARC 21 Bibliographic format
Incorporation of Former Headings into MARC 21 Authority Records
Indicating coverage dates for indexes in the MARC 21 Holdings Format
For further MARBI-related info see the minutes from the 2005 Annual MARBI meeting.