There is a chance (however slim) of a connexion with either Saint-Melanius (6th century Bishop of Rennes) or Saint-Mellonius (3rd century Bishop of Rouen). This would link Mellionec with St. Mellon’s, near Cardiff; St. Mellion, Mullion and Lamellion (Cornwall).
On the other hand, its origins may lie in W: meillion, ‘clover’ [B: melchen/melchon; C: mellyon ] which would link it with Cwm Meillionen and Maes-meillion, ‘clover hill’ and ‘clover field’ (Wales).
A last possibility is OB: mellhionou, ‘violets’ [C: mellyon; W: mellhionou]. What surprised me, however, is that the two are related in Cornish: it is an either/or as shown above. Maybe they are in (Old) Breton and (Old) Welsh too.