Some questions...
You've obviously put some thought into this. The thing I don't understand is the presumed proximity to Earth in both orbital radius and plane.
Also, how old would you guess an object would be. If it's old enough how could one tell the difference? Mathilde could've been a masive artifact that had accumulated debris and regolith and we'd never be the wiser without some serious excavation.
Do you have anything written up on this idea? Numbers, assumptions, that sort of thing...?