Pyethapp provides a simple hook to allow developers to plug-in user services that can intercept blocks and even create mini oracles in Python. Does Go Ethereum have something similar?
No. Geth does not provide any plugin infrastructure for custom code execution and there are no immediate plans to add it. We are aware that Python supports such features and we've considered it, but our code still is very dynamic and we don't want to introduce further APIs at this point that we would be committed to supporting.