While I've never actually seen any hard evidence showing that MMO can damage cats, I'm going to say that prolonged use of very high doses of it can potentially coat and damage a catalytic converter.
As Panzerman has stated, a single huge dose might not necessarily cause problems, but I personally would not want to risk continued/frequent overdosing.
I would say that using MMO in fuel at the recommended doses (2-4oz/10 gallons) should not have any negative impact on Cat performance, however.