You made me get up and go look at mine 🙂
The part didn't jump out at me, and I didn't find a similar part on any of the B&Ws I have here.
With that said, my initial thought was that it's a Zip drive blanking plate. All the ones I have here at home(plus, I think, my one at work) have Zip drives so if that's the case I couldn't find one because I've never had one 🙂
In addition, the design is different from the blanking plate used on later G4 graphite cases. The cases on those are effectively identical to the ones on the B&W G3.
So, is that a good enough non answer?