Is there are an API planned somewhere for that or is it going to be provided by the blockchain or wallet RPC API?
What do you mean exactly?
Network hash is currently displayed for PoW coins (and PoW+PoS coins) provided the wallet can give the information without being actively mining (not all do).
AFAIK none of the current PoS wallets provide PoS hashrate information, if you know one that does, let me know the API call.
I'd like to have that, but been wondering how to validate contributed tagging (to avoid spam and incorrect info). Currently you can post about a wallet in a coin's thread on bitcointalk and let me know (which means the community does the validation).