Well, in the first link you gave, you're not actually hovering over the divs that hold the buttons to make the buttons appear.
You're actually hovering anywhere over the entire div that holds your slideshow.
It's actually a mute point because of this, but what rtrethewey said about display:none, I'm pretty sure also holds true for visibility:hidden.
You can't fire an event by hovering over something that's either not displayed or not visible. Can't do it.
But, you can do something like this:
The .slideshow_holder div hides (covers) the .nivos when you're not hovering over it by having a z-index of 2,
and then 'scoots' below the .nivos when you do hover it by having a z-index of -1
Work with it, I'm pretty sure it can be done this way
You might be right in that once the .nivos DO show, you may have to additionally
have to have an a:hover on them to keep them showing (and therefore clickable) when you hover over them.
I thinking you might loose your focus on the .slideshow_holder when your mouse crosses over onto one of the .nivos