Not sure what option you have to get that fixed height handlepost lower.

If you replaced it with the adjustable height handlepost then you can obviously drop the upper section quite a bit lower. If the lowest position "out of the box" is not low enough, then you just cut some of the length off the bottom of the upper piece and voila, it goes in further and thus you have significantly lower handlebars. You could get REALLY low that way if you cut off quite a lot. For example if you were really really short, or you were some crazy TT guy. Hey wait...

i.e. you could get the bars down to the height where they would be just above the height of the lower handlepost section.
That's really darn low. Obviously you might not choose to be so extreme in your setup.