Mariann Asanuma, a fairly well known person with-in the community (She's a former Legoland California Master Model Builder, now freelance builder), made a large Lego Batman Mosaic back in 2008. From the MOCpages post about it, she says:
Hi all. Here is my most recent creation. Warner Brothers Interactive commissioned me make this mosaic for the LEGO Batman the Video Game Launch Party Event that was held at the Natural History Museum in New York City on September 23rd, 2008.
The mosaic is incredible with all the different colors and techniques, since it is, in fact, SNOT (Studs not on top).
Check out her blog here and the creation here.