What area are you in? I mountain bike frequently, but I'm not entirely sure what trails are "Cyclocross appropriate." I'm getting a single speed cross bike soon, so I suppose I would find out.
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I think Sycamore Canyon (Poway to Santee, CA) could be good if you skip the descent to Martha's.
San Clemente/Rose Canyons could be good give dismount and run across the river bed.
Florida Canyon (Balboa Park) is probably doable as well.