Why isn't there a way to label a column as "keyword"
If your goal is to de-normalize while mitigating the storage space requirements, Hybrid Column Compression in Oracle 11gR2 can help with this.  It is only available on Enterprise Edition with a few specific types of storage.  


  Storing column data together, with the same data type and similar
  characteristics, dramatically increases the storage savings achieved
  from compression. The database compresses data manipulated by any SQL
  operation, although compression levels are higher for direct path
  loads. Database operations work transparently against compressed
  objects, so no application changes are required.