Honestly, for the final site I'd recommend you do this in Dreamweaver. NEVER overlap slices. It causes untold headaches because of how FW handles slice export.
If you're just doing this for the mock up, you could do a series of disjointed rollovers, and use the same slice for each tooltip, while using a series of different states for the contents of the slice. It means your tool tips will appear in the same location for the mockup, but it will get the point across.
If you want the tool tip to appear in a different location each time, the fastest and easiest way to mock this up is to use multiple pages. One page holds the main "state" of the design, and as you click or mouse over a particular link, a new page in the mockup loads, with the tooltip in place.
I used both these techniques in my Fireworks Classroom in A Book to meet specific design/mockup requirements. The multiple states option gave me what looked like a Spry data table/ The multiple page option was used to emulate a series of pop up windows, where the windows were different sizes and in different locations on the page.
I found a way that seems to work well. This is only for a prototype, this is not intended to be a real web site or app.
What I did was just use a hotspot with a pop-up menu tiggered by onMouseOver. I fixed the menu's appearence to look like a tooltip (small text, yellow background). Then I put a location in the Link: property & it works great.
Hey, good workaround! Nice to see a use for the FW pop up menus.