Hi Aaron,
it's a tempting idea but I have to decline. The API is deliberately limited to the NIST interface. Once OpenSSL gains SHA-3 support we are going to use it in favor for the reference implementation. I don't expect OpenSSL to provide the full sponge API.
I also like to keep all options open so I can switch to a different and perhaps smaller implementation in the future. The reference implementation is huge and the binary is more than 400 KB. For comparison the SHA-2 384 + 512 module's binary is just about 60 KB on a 64bit Linux system.
Once a a new API has been introduced it's going to take at least two minor Python release and about four to five years to remove it.
But I could add a more flexible interface to Keccak's sponge to my standalone sha3 module https://pypi.python.org/pypi/pysha3 ...