We have tried everything, pushing feeds, setting up fields in "document dates" screen to enable all documents in the index to contain a value for Last-modified date. Nothing seems to make documents retain a "last-modified date" in the index.

How does one enable a last-modified date for all documents in the index?

Note: the meta value will come from an external xml source (not a database).

Followed the google instructions for our gsa version (6.4.0.G.22).

Yes I know the GSA version is quite old, we've been told by our google-representative support team themselves, that updating the GSA to the latest version "should" resolve the problem, and by "should" I mean, their GSA did the same thing (no last-modified date) and updating our GSA is another can of worms story :)