At the end of the day it is impossible to have completely anonymous games. If two friends in the same computer lab randomly find themselves playing in the same game, they will know who each other are. And this is true of real life too, America knows the Russians, the British know the French, we know which regimes we trust and which we don't. I don't think it is "metagaming" at all to treat certain players with caution when you have experienced their modus operandi previously. It's just common sense. And players who are continually deceitful, petty and generally unreliable will get the reputation they deserve, just the same as "good" players do. The "bad" players either learn to be deceitful in a more clever way, or they soon find no-one is ever going to ally with them in any game. Equally, if you have a successful and long-standing alliance with someone in one game, you are going to be more inclined to trust them in another. That's not cheating, it's just networking. Recognising and utilising another person's skill base.
I think building a reputation is and should be a part of being a good Diplomacy player - if everything is anonymous, then basically you're playing Russian roulette with your alliances, and the winner is the guy who guesses right rather than judges right.