in the database
find . -name "*.tcl" | xargs grep -i "demographics"
is probably the best place to start. I don't know the history of this table so it may have been used to a different extent in earlier versions of the ACS.
I would think you would want to modify your registration process with optional fields for entering demographics info (or a new page for entering info, depending on how much you want to put up front).
Occasionally and unintentionally (we presume), code customized for an ArsDigita client makes it way into the source tree. The data models for both the pets modules (www.pogopet.com) as well as the photodb module (www.photo.net) appeared briefly in ACS releases, only to disappear in a subsequent releases. (Yes, I know the photodb module has now released for general consumption.)
Since this question has been repeatedly asked on the web/db forum at www.arsdigita.com, one can safely assume that ArsDigita has no plans to release the Tcl procs that support the user_demographics table.
Another example is the geocentric bboard code, which was done for the pollution-tracking site Philip and others did about three (?) years ago. The code requires table contents and Tcl pages that aren't provided, and has never worked in any ACS Classic release. Knowing that, I didn't even bother rewriting the outer joins when I ported bboard over to Postgres...