Brace layers to designspace sources

Currently, brace layers are stored in an extra layer of another source (and linked to by the layerName in the designspace sources).

I wonder if it would make sense to put them into its own sparse .ufo files? That would make it easier to access them with other .ufo tools.

One thing that needs to be taken into account is that the new .ufo needs to be interpolated from the main sources to get the correct vertical metrics.

Any ideas about this?