I have been discussing this topic with Ming Thein on his blog (via his DP Quattro review), and a few people have informed me of few viewfinder loupes that can be used on the Sigma LCD screens. Here is a video to Brett Simison's You Tube channel where he talks about a Hoodman Loupe, and how to fit it.
Here is the link to the Sigma DP Quattro review and if you scroll down to the comments section, there are a few comments about fitting LCD loupes to the back of the Sigma LCD screens. (The Sigma DP Quattro has one available, but the DP Merrill series doesn't which is why we were talking about possible solutions. I live in Arizona so it's very bright here. An LCD Loupe would help tremendously on these bright, sunny days.)