Substitution variables in App.Config?

[EJ]

Is there a way to specify a substitution variable that can be used in
several places in the app.config file (like you can do in Ant/NAnt
build files)? For example, if there is part of a file path that I
specify in several of the appSettings key values, can I somehow specify
that path once, and just insert a substitution variable of some sort
(e.g., "${path}") and have it get substituted at run-time? I could
just separate them into different keys and handle it in code, but this
path is being set on a key in a custom configuration section for a
third-party component (log4net), so this is not possible. Also, I
tried to do this using XML Entities to no avail. Please advise.