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A field derotator needs to spin at precisely the right speed to cancel out field rotation caused by the alt-az mounting of your dob. It would need some means of calculating where in the sky it is looking to know how fast to turn, ie encoders on both axes.
You would also need to track the dob in both altitude & azimuth. This requires fancy electronics to coordinate the slow, smooth movement of three things at constantly varying rates to achieve a static field. Technically thus is possible - most modern professional observatories use this technique, but it's not a viable solution for consumer equipment. Meade do have a derotator for the LX-200 scopes, but I'm not sure if they are still produced and I've never seen an image taken with one posted on the web.
DT
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