I imagine that you would be able to store breaths in a nicrosil metalmind. Compounding that nicrosil would be interesting because as per normal compounding, it should give you more than you put into it. However, as Investiture is used to create positive gains in allomancy, you would need to be using Investiture to duplicate Investiture.
I'm not sure if or how this would work. Maybe Preservation's power would be combined with Endowment's? But then, if that's the case, would they still be Breaths? Maybe Breaths stored in a nicrosilmind get automatically converted into raw Investiture when tapped anyway.
If this does work, it could be an easy way to gain a lot of investiture rapidly. The Ghostbloods would definitely be interested in this.