Could be an interesting project - start off doing just a tailboard style with fixed front standard. I'd be a bit concerned about light-tightness though. Legos are not precision-machined, and I think it would be pretty easy to have light-leaks between the blocks, especially on the vertical faces.
I've made a pinhole out of LEGO about a year ago. Surely is a ***** to lightproof. I wanted something worthwhile and was kinda nice (aka didn't fall apart), so I gave up. But I came up with some cool designs of cameras!