I think that number in auto_publish.cache.php is a unix timestamp with the publication date/time of the next resource due to be published or unpublished. During every request, that number is checked against the current time. If the number there is less than the current time, the whole cache is refreshed and any documents due to be published or unpublished are processed. During the cache refresh, that number is reset to the pub_date of the next document after that due to be published or unpublished.
As far as I can tell, the only time that number in the auto_publish cache is updated is during a total cache refresh.
$modx->cacheManager->autoPublish() is the public method that resets it, but its argument is a fairly complex array. However, you can call $cacheManager->refresh() with an argument that I think would solve the problem with a minimum of overhead.
If you feel daring, you could try putting this code at line 219 of the CacheMaster plugin. It goes in the OnBeforeDocFormSave section, just below the complex $cacheManager->delete call and just above the break statement:
/* If pub_date or unpub_date are set, update the auto_publish cache */
if ($resource->get('pub_date') || $resource->get('unpub_date')) {
$providers = array(
'auto_publish' => array('contexts' => $ctx),
);
$cm->refresh($providers);
}
