Harlas, Dezza, Rowana, Ed, and even Dorg (though he's away at present) have their hands in the plot-work, so you can PM any of them.
That said, this sounds like something any GM can help with, if you have a character who is simply looking for blueprints to a siege weapon and then wants to build some. Feel free to PM me or another active GM and we can get the ball rolling.
