Hmm. Okay, let's think.
a) That's fine by me. Voluntarily weakening yourself in a situation where you may need it is something I highly encourage my players to do. Fate points are their reward for this.
b) Also fine. Part of the Fate economy.
c) This is a thing. The idea I had was that, essentially, flaring a metal would be an encouragement for the GM to compel that player into losing access to his power after a while. Pretty basic reasoning and utilisation of the Fate economy. Now I'm wondering whether a stress track would be more beneficial. The aspect is important, though, because there are a bunch of things that are only really compellable through it. Tineyes, for instance, have incredibly heightened senses when flaring their tin. They're also much more vulnerable to bright lights, loud noises and pain in general.
Perhaps it could work with a stress track.
Burning a metal gives you the standard benefits to your senses and running out is handled through compels. Flaring the metal creates the aspect and gives you a new stress track of X boxes that you can't use consequences for (it would have to be variable between metals; even flaring tin would not use it up at the same rate burning Atium does - maybe 2 for Atium, 3 for Pewter, 6 for Tin). One stress box is filled every round you are flaring your metal. The Taken Out effect of using all the stress boxes is that you lose access to that metal for the rest of the scene or until you Declare that you have more of the metal (or have a relevant aspect that you can invoke for effect).
Not entirely sure what level of effect that should have, however. I'm hovering between opening up 2 stress boxes for use and cleaning the stress track completely. I'm not sure which would be more balanced. Thoughts?