Existing Item Node Mapping should be extensible

Yeah, eh, I was in dire straits, as I said, so I just hacked [with the feeds module directory] "/plugins/FeedsNodeProcessor.inc" to have a case for UUID.

After

case 'guid':
          $nid = db_result(db_query("SELECT nid FROM {feeds_node_item} WHERE feed_nid = %d AND id = '%s' AND guid = '%s'", $source->feed_nid, $source->id, $value));
          break;

I put..

case 'uuid':
          $nid = db_result(db_query("SELECT nid FROM {uuid_node} WHERE uuid = '%s'", $value));
          break;

And, instead of hooking into hook_feeds_node_processor_targets as mentioned above, hacked the "public function getMappingTargets()" section to do the same thing.

I'm afraid I don't have time to cut a patch that would do what we want appropriately, but what you'd probably want to do is modify the existingItemId() function to implement something like: drupal_alter('feeds_node_processor_targets_processor_callback', $functions);, that generates an array with keys corresponding to targets (such as "nid", or "url", or in this case "uuid") and whose values correspond to a callback function that would process the target.

Then any module could add targets and a function to process them.

The GUID is unique to items imported via Feeds (if that makes sense).

So you have Site A disseminating a data Feed of some sort -- in my case, and seemingly yours, it's a feed of Drupal node datas -- one of whose data values is a UUID (as created by the Drupal UUID module).

Drupal Site B is passed the feed from Site A and, with your method, sets the "Feeds" module's GUID to Site A's node's UUID.

..That should work - your GUID can be anything. In fact, if Site B reimports Site A's feed later, as long as the nodes retain the same UUID, Feeds module should sync Site A Drupal UUIDs to the same Site B Feeds Module GUIDs, you dig? It would tell what nodes are the same and update instead of duplicating them (as long as Feeds is set to the, I think it's called, "Update nodes (takes longer)" option). Perhaps that's really what's causing your issue?

Why isn't your method good enough for me? Because I have a Parent site and a Child site which share the same codebase and share the same origin database. I need to be importing nodes to Site B based on Site A's UUIDs and then RETAINING the same UUIDs across Site B. That way, if nodes that existed in Site B [before the database branched off of A to form B] are updated on Site A, their updates propagate retrospectively to Site B.