Having had the problem of trying to turn large diameter workpieces from time to time, and not wanting to buy a very large lathe (which I don't have room for anyway), I've toyed with this idea several times. With a horizontal turntable and over-arm system, you can fixture a large diameter workpiece, then machine on it with a travelling 'spindle head'. It's a gantry mill, basically, but with a rotating table instead of a linear sliding table. You could mount something on it, like a flywheel as Cuemaker is having trouble with currently, and because it's horizontal, the flywheel or whatever just lays there while you take pains to center it. A jig or two helps you center it, then you clamp it down. The spindle head mounted on the overarm is under leadscrew control for horizontal positioning, and there would be a vertical range as well.
You have the options of rotating the table under power while using a stationary cutter (like a lathe), rotating the cutter only, rotating both, indexing the table, and supporting the center of the overarm with a removable post through the turntable axis (if the workpiece has a through hole big enough).
The turntable is basically a large faceplace, or a circular slotted milling table- whatever you would call it. It would have a hollow central shaft which is stationary, with a large bearing surrounding it carrying the table. The outer rim of the table could have support bearings as well to limit flex, or it could just be so solidly built that it wouldn't need that. I'm thinking it would be about two feet across, and probably have a maximum speed of about 300 rpm.
Normally, a workpiece that large in diameter wouldn't be very long, so a fairly short vertical range for the spindle head would not be much of a limiting factor. This could be the equivalent of a 24x10 inch lathe, or in some sense the equivalent of a mill with a 12 inch throat. If the overarm was able to travel over a range of about 12 inches in total, that would be roughly equivalent to a mill table having a Y range of about 12 inches with an X range of 24 inches. If you considered the overarm spindle to be the lathe spindle, you could mount stationary tooling on the turntable and use that as a sort of turret tool-holder. With the table held stationary and the overarm spindle stationary, the thing could be used as a shaper or slotter.
The worst use of it would be for long slender workpieces and threading, where a normal lathe of even modest size would suffice perfectly anyway.
Ok, so I either go down in flames, or you all will give me even more ideas and constructive criticizzleum.