Happy ending: the cat's owner called this morning and came to get him. Her, actually. She was still hissing her head off at us, and had only come out briefly during the night to jump on the refrigerator and hiss some more. When Ken got up to investigate, she lunged at him. This morning, she was sleeping in a corner of the closet but refused to let us get near. Her owner had no trouble, though, with nary a hiss as she scooped her up in a towel. She wasn't supposed to be out; her owner said she must have snuck out as she was leaving last night.
It was a rather poor night of sleep, between Moly trying to get into the bedroom and the gray cat (whose name we never did learn) hissing around. But we did do a good thing. I think.