Derailleur Configuration - Anyone ever seen something like this? Can't find anything on Google because I guess I don't know what search terms to use.

I am wondering if anyone has ever seen a derailleur setup like I have drawn in the picture. It is NOT for installation on a normal bicycle, but a normal derailleur takes up too much room below the gears and will not work for what I want to do.

(Using a small wheel in larger frame - I have lost all but 4" from gears to ground.)

Please advise on this 'central sprung gear' idea - if you've seen it, if it would work, etc.)

OH_ That three-gear center assembly would slide back and forth sideways on shafts to allow chain alignment.
I Designed wire and cable machines for about 10-years and a previous life and have indeed seen your setup many times as a system for maintaining tension on a cable; For that purpose it works great. If all you were trying to do was maintain chain tension at varying lengths, you would have something. 

But as Carel points out in the comments (and I've spent years testing this), You absolutely want the application pulley/gear to be as near the point of application as possible. Failing to do so will guarantee that your chain will walk up and down the cogs at will and will not shift when asked to. 

Your design would have to be modified such that the fixed pulley nearest the rear cogs would move in/out and forward/back as the gears change. Once you do that, you have something that looks very much like a modern derailleur (funny how that works). 

The truth is that a derailleur is a pretty elegant and simple approach that has been tested, proven, and improved over many years. Your best bet, in my opinion, is to play with the placement of a derailleur so that it is less in the way.