If you don't have anything constructive to say to a person and want to signal wishing their success I think good luck works just fine.
Either way, if the receiver is another "rationalist", they will understand, and if not, don't try to freak out the normals.
Is there any rationalist equivalent of "good luck"? I've tried a few variants, such as "work well", "knock them dead", "we're with you" and certain situation-specific phrasings, but haven't found anything that worked generally - though a hearty "may all the gods of Olympus be with you!" can serve. Not a vitally important point, but it would be nice to have something similarly supportive and yet accurate to say.