Prevent indexing of a folder without the built-in preferences

I know you can just use the built-in options to exclude a folder from
being indexed but how about if that folder contains some secret files
that you don't want someone else to know about? (Co-worker
conversations, files or applications from a new job you're secretely
considering, etc). Anyone could just see the GDS preferences and notice
that C:\My Files\Secret is excluded and get a little suspicious.

It'd be great to be able to place some sort of configuration file in
that folder that specifies that it shouldn't be indexed so it won't
show in the excluded list in Preferences.

Think of it as the Apache .conf files.