Just for completeness, I guess we should add
trinity --light --ropsten too.
See https://github.com/ethereum/py-evm/issues/612 for a jump-start on the read-only ones.
trinity attach - Attaches to a running trinity node
Are you thinking some system introspection to find the running trinity process and ask it where the IPC file is? Even that leaves some ambiguity if two processes are running. The easiest thing for alpha is probably to
trinity attach '/path/to/jsonrpc.ipc', although if that's the end goal, then we might as well add a command like it to web3.py:
web3py attach <ipc_file>, and support all nodes out of the box.