When has a risk-fleet strategy worked?

Are there historical examples of risk-fleet strategies working during an active war?

Yes, depending on the context. The High Seas Fleet's deterrent effect in the Baltic is one proposed example, and an earlier British fleet in being against France is recalled without a confident date.

One proposed example is the German High Seas Fleet's role in deterring British entry into the Baltic during the First World War. On that reading, its presence helped protect Scandinavian supplies, including food, important to the German war effort. An earlier Royal Navy example against France concerns retaining a smaller force in being until additional British ships could return and concentrate. The episode's date and reigning monarch are not recalled with confidence, so the example remains undated. A risk fleet's usefulness depends on the circumstances in which its surviving ships constrain the opponent.