I am no jeweler and I am certainly no saint, but I would suggest you consider St Nicholas' usual iconography, which I have just read to include the 3 balls familiar to all of us who frequent pawn shops, but also ships, a barrel, and a bishop's mitre. (Also a bald head and a short white beard, but I suspect those would be hard to work into a pin design.) You can look up for yourself what these things symbolize, other than St Nicholas.

I would work on either the barrel, or an anchor. I would add to this the Fleur de Lys. A simple design would be something made like a barrel with a Fleur de Lys cut out of it. I expect you might be able to incorporate an anchor and a FdL somehow. I like the association of the anchor with hope, and of course, with security... A FdL as a background, with an anchor superimposed might work, too.

Alternatively, you could place the three balls (bezants, if my memory of Sir Hillary Bray's heraldry lesson is good) on top of the flower. It might be cool to make the flower silver with the gold balls on it.

Do let us see what you come up with, if you will...