It's not clear to me whether the document that contains the long list of postal codes is a regular data document, or a reference document that contains the valid values for a keyword field, accessed via @DBLookup.
If the document is a data document, there's not a lot you can do, apart from changing the multi-value text field to a Rich Text field. That will hold all the data you want, but the values will not be available in views and agents. Breaking the field up into multiple multi-value text fields will probably not do any good, because the limit of summary data applies to the document as a whole, not just to each field.
If the document is a reference document, you can break it into two or more documents, each containing a subset of the postal codes and the lookup will still work, provided you don't exceed the limit of 64,000 bytes returned by @DBLookup or @DBColumn.
This was first published in June 2003