I would like to support the configSource attribute on my custom ConfigurationSectionGroup in order to allow for storage of the group contents in an external .config file. Unfortunately, I have no clue how to do this in a proper manner, other than writing custom code that loads and processes the external file. The latter is probably not a good idea because the logic is allegedly available in the .NET framework (as some of the built-in section groups, like <profiles> support it), though I don't know how to make use of it.

Did you ever find a solution for this? I want to do the same thing, but I haven't found out yet how to do this.