Wouldn't this concern be solved by a lightweight, non block chain client that connected to some other node that was maintaining and validating the full block chain?  Inv messages make transaction announcement very lightweight.  Also, if you want to monitor the network to assess the risk of a particular transaction never being confirmed (i.e. if there is a conflicting transaction floating about), you not only want to hear about the transaction once, you want to gauge how widely distributed and accepted the transaction is (so, you want to hear about it from a large percentage of the peers with which you are connected).