I don't know about Baldwin (Alec?) but as much as I enjoy Jack's Joker in this, I'd have *loved* Mark to have got a shot at a live action Joker.
That said, watch him in The Flash (1990's series) as "The Trickster" and you can pretty much see how he'd have played it. It's an awesome performance (predating his Joker voice work, AFAIK) and easily the standout high point of the whole series. Also, it's clearly where the idea for Harley Quinn came from, in the form of Trickster's sidekick "Prank". Although different to his later Joker vocal performances in many ways, Mark as The Trickster is the closest I feel we'll ever get to how he would have done Mister J in the late 80's-early 90's live action.