Softimage said that they wanted to implement a temporary replacement feature consisting of a shaderball-like approach, but there is nothing like that in XSI 3.0.
Perhaps later as an add-on.
Softimage unmapped the Node Preview feature because there is a stability issue with it. However, I found that remapping it to a keyboard key (like P) doesn't cause any crash, unless with heavy scenes and changing connections while the region is refreshing.
Hope this helps
Salutations - Cheers
Bernard Lebel