Thanks to concise advice from Owen Winkler (aka ringmaster), my test Habari install now has the same URLs as my existing WordPress blog. That means that when I move, all my links will still work. It would have been a pain to redo all my internal links, but those three sites out there in the wild web that link to me are really valuable ...

For reference (lines wrapped for clarity):

INSERT INTO habari__rewrite_rules
    (name, parse_regex, build_str, handler, action,
     priority, is_active, rule_class, description)
VALUES ('display_entry',
    '%(?P<year>&#92;d{4})/
       (?P<mon0>&#92;d{2})/
       (?P<mday0>&#92;d{2})/
       (?P<slug>[^/]+)[/]{0,1}$%i',
       '{$year}/{$mon0}/{$mday0}/{$slug}',
       'UserThemeHandler', 'display_post', '8', '1', '0', '');

[Update: Don't copy and past the query above, you'll get spaces in your regex that break it. Use this text version instead. Also, make sure you change the table name to the correct prefix, habari__ is the default and most likely.]