Wouldn't it be safe to just always use multiple hashes (e.g. SHA-2 and SHA-3 and Whirlpool) and XOR the results to generate one final hash? A weakness in one of the algorithms would not weaken the combined hash. And even if *all* hashes would be effectively broken, it would still be far from trivial to forge arbitrary data that results in the same final hash.

Oh, really? Can you post just ONE example of a 512-bit hash collision? I mean two different pieces of data (no matter how long or short) that have the same 512-bit hash? Or even just 384-bit perhaps? 256 maybe? 224? 160?