There is the tree structure, and there are tags. I can't think of any easy way to mix them. Even in a custom snippet that retrieved both, I think you'd need a custom sort routine if you want to order them in any way.
If you don't care about the order, a custom snippet might not be that bad. It's just a matter of retrieving the resources, then looping through them using something like this:
$tpl = $scriptProperties['tpl'];
foreach ($docs as $doc) {
$fields = $doc->toArray();
$output .= $modx->getChunk($tpl, $fields);
}
Of course that won't give you all the formatting features you get with getResources().
