With rebar.config and *.app.src you can make use of dynamic configuration based on file:script/2.
If a <name>.script exists in the same directory as the original file (in the case of rebar.config that would be rebar.config.script), the script file will be evaluated and the result used as configuration.
For convenience two bindings (variables) are available in a script during evaluation:
CONFIG - result of file:consult/1 if the script file being evaluated also exists  without the .