Are trace amounts of lead in seasoned bakeware safe?
I, and in general we, cannot answer if something is safe.  There are instances where we can say something is clearly unsafe, but when dealing with things like is this piece of metal safe for cooking we are not equipped to give anything but anecdotal answers and partially informed opinions.  

The only safety answer that is actually reliable would come from lab testing and that is why we shell out tax dollars and public funding for organizations like UL and USDA in the US and similar organizations in other countries.  They do testing and declare metal products, glass, plastics, etc. to be food grade or not food grade according to their composition and reactivity to normal food products.  They are not always correct, but their declarations are made from testing results, not guesses and partially informed opinions, mine or anyone else's.

Also an opinion, but one that I think is really the only prudent and reasonable one to make is get a product that is rated food grade, not a custom made item you or others think is safe.  The amount of toxins in the metal may well be low, well within standards, but that does not say it is not in a reactive form which will enter you food on first contact with a slightly acid item, and lead may even be one of your least concerns.  A piece of steal which was not manufactured under food grade conditions could have many other toxins that are not permitted in food grade manufacturing, permitted because they are in low amounts that are safe for the intended use but become readily transferred to food.