Doesn't matter if it's MD5 or SHA512. You can extend either hash function in the exact same way (http://en.wikipedia.org/wiki/Length_extension_attack)
The documentation for both Keccak and BLAKE2 recommend prefixing a fixed-length key to the message to do MAC, pretty much like in the vulnerable example, but with a better hash function.
I think you meant SHA-224/384. Both SHA-512 and -256 are vulnerable to length extension because their internal state is dumped and resumable. With SHA-224/384, you only get a truncated state (from 256- and 512-bits respectively), which you can't pick up and resume.
I wouldn't consider SHA-224 immune to length extension since it only truncates 32 bits, which is low enough to brute force.
[1] http://csrc.nist.gov/publications/drafts/fips180-4/Draft-FIP...