The best way to do this is to do it manually
Plan your ‘damage’ and divide the mesh accordingly. It’s not fun but you will get the results you want.
If you are looking for ‘quick fix’ solutions in XSI you could try the ‘Shatter’ script, I think it can be found on XSI base.
Blastcode is a terrific plug in for Maya (no idea about XSI versions) but it might be too overkill for what you want to do?
An alternative way to generate shatter effects is to try the free version of Houdini and use the Shatter (SOP) then export the results as an .OBJ.
However, most people who have tried this technique end up resorting to the old fashioned geometry cut,chop, extract, extrude and bevel methods.
I’m sure ICE could offer some sweet method to problem but I have had no time to delve into it yet