Figure out what you actually mean then just say that. My suggestions are "may you succeed" or "I hope you prosper".
"I know that nothing I can do can affect the likely factors you are likely to face, nor your attitude and aptitude when facing them, but I wanted you to realise that I am sympathetic and supportive, in the common knowledge that this will make you feel better, no matter how objectively irrational that seems, therefore I will take the time to make you feel better, because I like it when you feel better".
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.