Regtest: address is still at zero after sending some bitcoins
getreceivedbyaddress is only able to show the balance for the addresses in the account: it stores only UTXO for these. The addresses generated with Pybitcointools are not stored in the account (contrary to addresses generated with getnewaddress).

To get the balance of any address, a block explorer is needed, which scan ALL UTXOs of the blockchain (like blockexporer.com or blockchain.info). 

And for Regtest, it's more complicated since a local server is needed like https://github.com/bitcoin-abe/bitcoin-abe.

Conclusion: I"ve finally switched to Testnet mode, for convenience.