I guess that Download is the Rodney Dangerfield of modules -- it just doesn't get any respect. There isn't even an entry in the SDM for it!
The answer, well, an answer, is to change the first few lines of $server/www/download/download-add-version-2.tcl as follows:
From:
{scope}
{group_id}
{user_id}
to:
{scope ""}
{group_id ""}
{user_id ""}
and
release_date
to
{release_date ""}
Truth is, I'm not sure why it works, just that it does.