Here's a rough draft of the network protocol specification. I'm calling it RFC 1 (RFC stands for Request for Comments).
NullpoMino Protocol 1.0 Specification
This is the promised table of opcodes that I've come up with so far. I left some room in the game opcodes area for expansion, and I feel like it would probably be best if we could finalize the design of network games and their communication before this is set in stone.
NMP opcode table
When this proposal is acceptable, I'll begin work on a reference implementation server which uses the protocol.