I'm sorry, I know this is a bit of a "snarky" type reply, but I don't really understand what you are driving at with your question. Without more constraints on the question, there's not really any point in searching for the minimum solution.
Any ideas about how to better specify the rules of the competition to exclude degenerate answers like the one you provided? The goal is to answer the interesting question. You'd never find such a high-level instruction in a real instruction set, because it wouldn't be useful except to play Chess.
I'd add a rule like "The goal of your instruction set is to be useful while still implementing Chess," but then that would exclude answers which give interesting instruction sets that aren't necessarily useful except in implementing Chess in the fewest possible bytes.
Thanks for pointing out the flaws in my ruleset. I've been toying around with creating a blogpost about this, so having a bulletproof ruleset or at least one that isn't so easily sidestepped would be great.
Incorrect. A single instruction can be Turing complete.
http://en.wikipedia.org/wiki/One_instruction_set_computer
Though the relevant question is whether universal computation can be built on the back of an instruction which plays chess rather than one of the more typical bases for a OISC.
What are some typical bases for OSIC?
Well you can have two instructions: SUBLEQ and CHESS. Then it's Turing-complete, and there's a one-bit program that plays chess.