I'd suggest PM'ing vik. He has done countless reviews on bike gear and uses trekking bars on some of his bikes. He posts here and in http://forums.mtbr.com/ and I'm sure plenty of other sites. He's very helpful and knowledgeable and usually posts in Commuting and Utility...that I've seen.
Here's his blog:
http://thelazyrando.wordpress.com/?s=mirror

I believe there are MTB bar ends that you could add with your Ergons that would also permit the addition of some of the other common bar end mirrors. It'll just take some searching. I'm not a big fan of the bar end mirrors...too much vibration. I just keep my head on a swivel. Although I have considered getting one of these.