Why can't I use property expansions in Auth Repository? Is there a workaround?

I was hoping to use hardcoded OAuth tokens in my project, but protect them somehow so they aren't clear text in source control.  I was hoping I could use Custom Project Properties that were read from a file that was ignored by source control.  This doesn't appear to work.  Property expansions don't appear to work here.  Does anyone know a way to do this?