That is a great question. Here's a description of how the Align commands work, from the online manual for Fireworks CS5:
Left: Aligns the objects to the leftmost selected object.
Center Vertical: Aligns the center points of the objects along a vertical axis.
Right: Aligns the objects to the rightmost selected object.
Top: Aligns the objects to the topmost selected object.
Center Horizontal: Aligns the center points of the objects along a horizontal axis.
Bottom: Aligns the objects to the bottommost selected object.
As you can see, they pretty much work by extremes and averages instead of by "key object". A good way to visualize this is with the Scale tool (Q). Select both objects and apply the tool: the resulting bounding box shows you the boundaries within which the Align commands work. The centerpoint is the vertical and horizontal axis to which center align commands will adhere.
Unfortunately, the "key object" concept is not available in Fireworks. And I'm surprised that no one has created a Command or Panel extension to address this. (Have you tried searching?) Don't be shy about placing a feature request with Adobe for this, as it seems long overdue:
One way that you might work around the limitations of the Align panel is with the use of guides, though this would be harder to do with a centerpoint alignment than with an edge alignment. The basic idea would be to draw out a guide for the edge or centerpoint to which you wish to align, then proceed with a normal Align command. Then move both objects by hand back to the guide that marked your original "key object" alignment. (View > Guides > Snap to Guides might help with this.)
For a center alignment, in particular, there are two panels that might be of use: Trevor McCauley's Transform panel and Eugene M. Jude's Guides panel. With the Transform panel, you could select the "key object" and view its center X and Y coordinates. With the Guides panel, you could select the "key object", and then draw a variety of guides with just a single click. (I think the Guides panel would make a good choice in this case.)