I never considered Howland using poison against Arthur Dayne, but I suppose it's possible. As has been made abundantly clear, we don't know enough about Howland at the time of the events at the ToJ to say how good a fighter he was and how he would have fought against Kingsguard. Maybe he would have used poison to even the odds. The way I picture it, however, is that he somehow distracted Dayne long enough for Ned to kill him. It's possible that it wasn't an epic fight by any means. Maybe Ned and Howland just got lucky. That's more GRRM's style.