Actually, expanding on this idea, a simple public website akin to blockexplorer could offer this service.  For a given transaction, it would report how much hashing power was working on the requested transaction.  Assuming pools do not reject a given transaction and relay transactions that they are working on, this would be done by simply requesting current transaction data from specific pool operator nodes to check what transactions they have seen.  By estimating total network hashing power, it could then calculate a propagation metric (50% propagation = 50% of hashing power has seen transaction).