But N20 is something we really would like to do one day... just want to run it down a drag strip once!! 
[/quote]good call, didn't think you were taking the piss when you mentioned N20, will enjoy following the progress.
the question I had in my mind was covered off above, how would it get from the turbo down to the sump, the only thing I could think of was via the oil ways in the head, looking at the head I have on the bench they are pretty small and the route isnt so clear so would have thought any fragments would possibly lodge themselves in the oil ways rather than get down to the sump, but if the part you have is small, then maybe your right.
if it looks like a bearing and one side is worn through to the brass then that to me means the bearing is well worn, if it has fragmented I would have expected there to be some knocking noises from the area its come from. having had a few big ends go in the past I know what they sound like
guess you will want to be sure the engine is up to spec before you start pumping N20 into it, last thing you want is a piston coming out the side of the block aye...
That head is fubar for sure!! 
What the heck did he do??