Long story short, this works:
Foo ret = new Foo();
Trace.TraceInformation("Doing stuff to

.", ret.ToJson());
ret = collection.FindOneAs<Event>(someQuery);
return ret;

Take away the ToJson() call it breaks because BsonClassMap.AutoMap() does not get called which causes BsonClassMap.LookupActualType() to fail. I am not sure how to fix.

Let me know if you need a better description and I can provide one.