Why is this an annoyance? How else can you look at a painting and get any feel for it? What would be the point of looking at the original work from a distance that kept you from appreciating it? I think most museum patrons understand this. I never step in front of others of course but usually move to unoccupied works like you mention or else wait for the crowd in front of a piece to disperse so I am comfortable I'm not blocking the view of someone who is already there. Have you had someone say something to you about standing too close to a piece? Was there an ensuing melee?
John Thornton