We do have the form 'oughtn't' - so this could be used as a tag where an opposite way tag is used.
We don't tend to use this form very often in modern English, possibly because it sounds a bit pretentious. Most grammars advance 'should' as an acceptable alternative.
I'm a bit confused by your example though. Does Bob know that there is going to be an accident? I think I'd change tense a little.
'Bob ought to have let me know about the accident, oughtn't he?'
'Nothing...' = not a thing = not a single thing...so the tag should be '...did it?'