When you say formal, do you mean black-tie? If your intent is to wear the sporran with the jacket you describe and a four-in-hand tie out to dinner or an evening on the town...I would say it probably would be fine.

On the other hand, if what you mean is a tuxedo equivalent evening...I, personally, would go with a fur sporran with a chrome cantle. Again, that's just me.

Also, who are the others you would be with are in the "formal" setting. If you are the only kilted person, you will be able to pull off whatever you wish more easily (and possibly more comfortably, since no one will know any better). If you will be going to an event with mostly kilted people, I would err on the side of caution and go with a formal sporran.

I am sure there are others here with better understanding of the appropriateness, but that would be my starting point.

Best of luck!