Genesis theme showing past events on default archives

I’m attempting to set up an agenda view using the default archive for events at mysite.com/events (this is a Genesis child theme).

It mostly works great! The events pull in, and the category links also pull in events on the appropriate category. I had two issues: 1) the events were not using the correct order, and 2) past events show up in this view.

The first issue I resolved by modifying the query:
https://gist.github.com/jonschr/c26852dbe62fa3ec251a9a4466e53836

The second issue should be resolvable by modifying the query in a similar way. I see what I need; it’s the ‘start’ column – I want to do a meta query which compares ‘start’ to time(), essentially.

Something like this added to the pre_get_posts hook:

$meta_query = array(
     array(
          'key' => 'start',
          'value' => time(),
          'compare' => '>'
     )
);
$query->set( 'meta_query', $meta_query );

However, I seem unable to do this sort of meta query because the information is in a nonstandard table (it’s not in the post_meta table, but rather in the sc_events table).

I wondered if you might be able to point me in the right direction.