3) Miner starts a request to long polling URL with GET
So yes "" does make sense because the pool can just check wether it's a GET or POST request...
ANY client that EVER uses GET for JSON RPC is wrong. Read the HTTP spec, GETs should never have message bodies. DiabloMiner follows the HTTP specification and uses POST for all requests.
If you want LP to work, it must be a unique URL.
Try running DM with -dd, you'll see a matching flood of LP returned messages because your server is not holding the LP request open correctly.