This is a bit unrelated but still on topic. Joist hanger have a purpose other than joist. It has been suggested they can be used in old mortices as reenforcement. As in a tie beam with a principle rafter where the quality of the wood at the relish is in question.

I take it that it is not the bare metal which is the problem with the black stain and deterioration of the wood but that the metal has different properties with temperature swings and moisture condenses on it repeatedly over time, rots the wood. For these new fangled coating on fasteners time will tell, we just have not had enough time. So is the coating there to keep the fastener from corroding or the keep the wood from rotting? Will you still get a rotting action around a screw over time, it just will not be black? Hangers are galvanized.

There is a lot of steal use in old timber work, from queen rods to small 3" dogs.

I would not feel comfortable using lags to tie the tie to the post, even toe nailed. 1" at least on the housing. I have seen a heavy 1/4"x3" strap used on top of the tie, say 3'+ long and running through a hole in the post and nutted on the outside face of the post. Weld a rod or black smith a round shaft with threads. Low tech, simple and it addresses the issue.

Tim