Differentiate between a feature being disabled vs absent

In our hosting-features registry, we cannot currently differentiate between a module being disabled, and a module being absent. If we instead used the feature name for the key, and the value was a simple toggle, we'd be able to do:

$features = drush_get_option('hosting_features', array());
if (array_key_exists('this_feature', $features)) {
return $features['this_feature'];
}
else {
// react to missing feature...
}

Alternatively, if the value for an enabled feature was set to the feature name (instead of '1' or TRUE), our current in_array()'s would still work too.