It seems to me that Seirawan Chess is just one special case of Pioneer Chess invented by Mats Winther in 2009.
No, there is a difference between the rules. In Pioneer Chess, you must place each additional piece behind the piece whose space it may occupy after the piece moves away before play begins. So, each additional piece has only one space it may be dropped on. But in Seirawan Chess, each additional piece has eight possible spaces it may be dropped on, and where it will be dropped is determined during the course of play, not before play begins.
No, there is a difference between the rules. In Pioneer Chess, you must place each additional piece behind the piece whose space it may occupy after the piece moves away before play begins. So, each additional piece has only one space it may be dropped on. But in Seirawan Chess, each additional piece has eight possible spaces it may be dropped on, and where it will be dropped is determined during the course of play, not before play begins.