Associating comments with special pages/module output
Well, I think you pretty much have the answer. 

Handling comments without the help of the Node API would probably be a lot of work.

But if you don't need all the extra features the Node layer brings, you can try to apply the same method on an entity. The Reply module takes care of attaching comments to your entity type. 

Again, it's more work than simply using a node to store each item comments section. Just use your callback function to load or create a node with a field storing the item ID (if it doesn't exist yet on the database).

You can then use drupal_goto('node/' . $nid, array(), 301) to redirect the user to the comments page.