i have a "monovue" viewing filter from a uk company called SRB. It is a deep almost olive colour, but it is remarkably good at reducing the colours in a scene to tonal shades giving a fair idea of what the scene will look like in monochrome. THat said, it is too dark to use in subdued light, and i find i have, to some degree learned to "see" in mono now anyway so dont use it a lot. It is sometimes helpful as an aid to choosing a filters for contrast though.

As for composition, I find the general area I want to photograph, then i use my viewfinder, and walk around until I find the shot i want. I dont think this would be overly practical with the LF cameras though!