Derailing the drive chain

Okay, I was having my after dinner coffee and looking at my bike and especially that butt ugly chain tensioner. The most unattractive part of the machine and mentally designing a better looking one when I ran across my old front and rear derailers in a srap parts box. I started playing with the rear one and thinking how I could use it as a spring loaded tensioner when I came up with a thought.
Could I stack two sprockets, spaced apart, on the rear wheel, attach the derailer with either a wheel or a sprocket and shift gears by derailing the drive chain over to the side just the width of the chain or enough to engage the extra rear sprocket?
Has anyone here ever tried it? In my mind I can see it working with a little modification to the left seatstay to clear the chain...but it should work. Chain alignment is critical but moving it a chain width that far back shouldn't compromise the chain's running on the engine sprocket. These chains have enough flex to allow the slight misalignment necessary.
Any comments, suggestions?

Since my first post on this I've been out in the garage scheming and doing some engineering. The tensioner / derailer would need to be spring loaded to keep a constant tension on the chain as it moved from the larger sprocket to the smaller and visa versa. I have that part worked out, at least in theory. I'm thinking of a sliding shaft that will hold the rolller or sproket activated the same way Sturmey Archer used to do their old 3 speed hubs; with a cable that would pull but the movement would be lateral to move the roller/sprocket in and out the required 1/2" or so.
As for peddle starts that shouldn't change. I'm not sure about on-the-fly-shifts yet but just having the option of a lower and higher gear would help in many of my riding areas with hills and flat terrain. Winter is setting in here so this will be a winter project. I'll keep you posted on my progress.

Yeah, Tramp, I know they have shift kits available but the ones I've seen used the bike's existing cassett and derailer and it uses bicycle chain for the main drive. I can't help it but I feel that's asking for premature chain, hub and sproket wear and failure. Those parts were never designed to take the load of an engine with it's power pulses and stress. The larger 41 chain I'm using and matching sprockets could handle it if I can get the chain to move as I envision it doing. Besides, if my idea works it won't cost me anything. That's about all I can afford now that I'm only bringing home less than half of what I used to. Hard asking the wife for bike money when she's still working full time and carrying most of the financial load. Retirement isn't near as much fun as I hoped it would be.