Are generator "bonding plugs" safe and legal?
Are you talking about just plugging in a plug with no cord, that internally connects Neutral to Ground?  I imagine it would work perfectly... until someone who doesn't know what it's for assumes it's a simple outlet cover and unplugs it to plug something else in to that outlet.
As far as whether it's code-legal, probably not.  Code tends to frown on things that make it easy to create a dangerous situation inadvertently, which this certainly would do.  The electrical code (AFAIK) doesn't say anything about this situation directly though -- instead, the code says you must use equipment that is UL Listed (or listed by some other NRTL, such as ETL), and that you must follow its instructions, which were part of the UL listing.  If your generator says that you can bond neutral and ground with a &quot;bonding plug&quot;, then it's legal to do so -- but you probably won't find any that say that, because I don't think UL would approve a bond that can be removed during operation.  Instead, the instructions will probably say how to open up the unit and bond N to G internally, or it will say that you must use a 4-wire feed to a panel that bonds N to G already.