This one is not a strict frequentative. Muzzle derives from Old French musel and/or museau, (ca.1210). Its origins are obscure: from low Latin musus via various old French variants such as Gascon mus for face, or Béarnais mus for snout or the face you pull when you're in a bad mood ('grouchy face'?), to which a possibly adjectival ‑el had been suffixed. But... Latin snout, musus, is not a million miles from Latin mus, mouse, and, after a couple of drinks, one might start wondering whether musus came from something looking like a mus attached to the front of a face... If musel / museau do(es) derive from the Béarnais, where, as in Latin ‑lis (‑lem), the ‑el might have switched role from attribute to adjective - in other words represent a state and therefore not a frequentative - prefiguring the ‑el pretending to be one in the French verb museler, to muzzle, which could thus perhaps be a crypto-frequentative.
An informed reader will see that I'm out of my depth here. But I'm not here to sink or swim, I'm here to learn and find out, so expect updates anon!