It seems to me that Seirawan Chess is just one special case of Pioneer Chess invented by Mats Winther in 2009
Musketeer Chess is a version of Pioneer Chess. Except that it allows introduction of two pieces, rather than just a single one, through the Pioneer-Chess gating mechanism.
Actually I like the Seirawan gating mechanism better. It doesn't require a game prelude governed by additional rules, and offers an interesting dilemma for when to gate the pieces: a piece that still might appear anywhere could be worth more than a piece in a known location that you currently do not need yet, but as the number of gating opportunities decreases during development of the other pieces such an advantage decreases, and turns into the risk that you might not be able to gate at all.
Musketeer Chess is a version of Pioneer Chess. Except that it allows introduction of two pieces, rather than just a single one, through the Pioneer-Chess gating mechanism.
Actually I like the Seirawan gating mechanism better. It doesn't require a game prelude governed by additional rules, and offers an interesting dilemma for when to gate the pieces: a piece that still might appear anywhere could be worth more than a piece in a known location that you currently do not need yet, but as the number of gating opportunities decreases during development of the other pieces such an advantage decreases, and turns into the risk that you might not be able to gate at all.