Mouseover effects for a single cell can be done similar to the row highlighting with the help of two global functions (see /lib/system/html.js):
function mouseover(element, name){...}
function mouseout(element, name){...}
The first one adds a CSS class selector 'name' to the HTML element, the second removes it.
To create mouseover effect you have to define a CSS class:
<style>
.active-cell-highlight {background-color: green!important}
</style>
and add the following script
var column = new Active.Templates.Text;
column.setEvent("onmouseover", "mouseover(this, 'active-cell-highlight')");
column.setEvent("onmouseout", "mouseout(this, 'active-cell-highlight')");
obj.setColumnTemplate(column);
You can combine both row and cell highlighting: