I admit that I laughed when I saw the Sigma DP1 Quattro press photo showing it with a boxy viewfinder attached to the LCD. It's simultaneously absurd and brilliant. It's a bad solution to a definite problem, a workaround hack brought to you by the very people who should have designed the camera better in the first place. Bulky, awkward, expensive, difficult to use, poorly implemented, and yet grudgingly effective: it is a pure distillation of the very essence of the Sigma DP series.
And yes, I routinely use an aftermarket EVF loupe on my Sigma DP3 Merrill.