The engine protocol has been finalized: any further changes will be either backward-compatible, or will only apply to engines which indicate that they support newer protocol versions.
In other news, I have the following official, committed entries:
1. Me, and OpenTafl's traditional alpha-beta/evaluation function AI. (OpenTafl will only play in the round-robin stage for reasons of fairness.)
2. My friend Shenmage, and his general-purpose neural network AI.
3. ags1, from the java-gaming.org forum, and a chess-style rules-based pruning/evaluation function AI.
If you're in-for-sure, let me know—I want to update the tournament page with who's going to be involved.