If you mean something beyond the standard chainring guards then whilst I'm sure its been done, I've not seen one. The radical change in the geometry of the chain position as it folds makes it hard. If it were me I'd start with the front section of a Hebie Chainglider and cut it down a bit I guess. There's plenty if places online that sell them and the kind of good LBS that the older Brompton dealerships tend to be are likely to have seen them in action.