This may not be precisely what you ask for, but my colleague Paul Gunnells wrote a nice article for the May 2006 AMS Notices with some color pictures involving Kazhdan-Lusztig cells in small rank Coxeter groups. The link is http://www.ams.org/notices/200605/fea-gunnells.pdf
I have had some conversations with him about cells and have gotten some insight from the pictures he generated using graphics software (which probably wasn't Mathematica but also doesn't run on his current computing hardware). This has just been a sporadic interest of his, though he has an ongoing collaboration with Belolipetsky mostly involving cells in Coxeter groups which are not finite or affine.
Of course this kind of Coxeter group information is only one part of the building construction, but a crucial one to visualize.
ADDED: To follow up Greg's comment, there have been other items in the Notices dealing directly with Bruhat-Tits buildings. An article by K.S. Brown in the November 2002 issue has a nice $A_3$ picture: http://www.ams.org/notices/200210/what-is.pdf
(Most such articles and graphics have been influenced by the editorial work of Bill Casselman at UBC, who would be the best source of technical information.)
