Sure. In the bottom left, enter a negative number for the Z position. The center of the zone will then be at a negative number.
WC
EDIT: I did some expmeriments and the client is *definitely* reporting your loc based on your eye level. Try running around as a big race and a small race and looking at your /loc z coordinate to see what I mean. I then dug through the server code and it sets the mob's loc to equal your loc. To me this looks like a *big* mob like a dragon should always appear in the ground regardless of the zone. I'm unsure why it works in some zones and not in others.
I'm not familiar with the #spawn command or how to use it. Could you post a #spawn command that's giving you trouble so I could try it in various zones?
EDIT #2: Okay, I've been testing with various #spawn settings (client macros are indeed terrific) and the problem seems to be that mobs don't fall at all. If you go to West Commons and do #flymode 1, then go up into the air and #spawn a mob, it falls to the ground. This doesn't happen with OpenZone zones, however. I'm still trying to find the problem. I've found and fixed three other problems in the zone export code trying to track this one down, but I haven't gotten it yet. I'm looking at the problem right now, but no guarantees