This may not be useful for answering the "hacked a (non-computer) system" question on the YC application form, but I think a script which can run on a linux system and construct a bootable FreeBSD image qualifies as a cool hack nonetheless.
Want to try my Exorcist script version 6.66 that turns a FreeBSD system into a Solaris system?
Not particularly, but I love the idea. The depenguinator isn't really about making people switch from linux to FreeBSD; rather, it's about allowing people to install FreeBSD where they otherwise would not be able to -- most commonly, on servers rented from cheap dedicated server hosting companies which support linux but don't support FreeBSD.
Did they ever fix the security flaw you found?
Who is "they", and which security flaw are you talking about? I've found several over the years...
Some linux distributions disabled hyperthreading by default as I recommended; others didn't. Some cryptographic libraries have fixed some of the more obvious ways that this can be exploited, but no library is completely immune. Intel hasn't done useful at all.