By now, the tooltip text doesn't use the full range of supported html tags of the flash TextField. By now it is impossible to set a part of the tooltip body in bold font, as shown in the following example:
$pieValue-> set_tooltip("$label<br><b>$percentage%</b> ($value$unit $labelName)");
The reason for that can be found in Tooltip.as in the function make_body (line 166-182). The current behaviour is to parse the CSS definition by hand and applying the single properties to the whole TextField. Instead you should use the styleSheet property of the TextField.
I clearly see the reason for the customized CSS parser, since the flash CSS is very different from common CSS and lacks support for many tags or introduces property replacements (like "leading" for "line-height"). I think the best way to get the best of both worlds is to translate the custom CSS class back to flash CSS and applying it to the TextField.
Please have a look at my patched version of Tooltip.as.
patch