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Think of a Herschellian scope. Only an off-axis portion of the mirror is used. So if you had a 4m mirror the effective diameter would be 2m. By altering the size of the secondary you are not obstructing the incoming light path which means that you could intercept the light cone closer to the mirror and deflect the focusing rays to a convenient position, ie. steerable (a little). The abberations would be calculable and if you were imaging the image could be cleaned up with an algorithm. This could give you a usuable field of maybe around 40° centred on the zenith. Hmmm ....
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