It does take a very long spring line to rig it the way I suggested, but there's nothing wrong with that if that's the tool you need to do the job. In his first post, Sabreman said that, when he rigged it the normal way, the wind blew the boat back alongside the dock before he could go to the bow, release and retrieve the spring line, and get back to the helm and in control of the boat.
If he can release and retrieve the spring line without leaving the helm, then he can use the engine to hold the boat in position until he can retrieve the line.
Floating lines are commonly used whenever there is a chance they might trail in the water near the prop. For example, they're often recommended for use as dinghy painters, and they're used on Lifeslings, and similar devices, so they aren't likely to get tangled in the prop. In this particular application, you're not backing down on the line. When you go forward, the line will trail alongside and behind the boat, especially if you keep your boatspeed down until you retrieve the line.