You could misuse the Alt tag to do this. An Alt tag is an alternative desciption of an image and is used for accessibility, among other things.
You can also add a tooltip to your hyperlink, but adding a tooltip to the hyperlink if an Alt tag for the image is already present can become problematic, although it is possible.
Here is an example of a hyperlinks code, where the link reads Home and points to a page called index.html:
Now, here is the same link, but with a pop-up tooltip that appears as text when the mouse cursor hovers over the link:
title="Return to our home page"
Simply adding title="" into the hyperlink code gives you the text message, which can obviously be altered to suit your requirements.
I'll be in front of a machine with version 7 of NetObjects tomorrow morning, so I'll have a closer look at the available options. The notebook I'm typing this on only has versions 7.5 and 8 installed and, being a naturally lazy hack, I'm not going to crank up another machine in a different room at this hour.
Assuming the title="" is a little undergunned for what you'd like to see, I'll take a look in the morning and see what can be done.