Interchangeability of hash outputs
If it's replaced by another function with the same length output, then I assume tweetnacl will use its output just as happily?


Yes, it seems to be a normal collision-resistant cryptographic hash function, replacing it by any other will not significantly alter the security properties (unless one of them is broken) but will of course break interopability.

In fact if you could find a security-relevant difference between two secure hash functions (given only oracle-access) at least one of them would be broken.