With the new features in 2009.2 you can do this pretty easily. You'd make a tagger that provides IClassificationTag objects. Then you'd make an IClassificationType that is registered with a HighlightingStyle that does a different FontFamilyName or sets Bold = true, etc. Your IClassificationTag's would return an instance of that IClassificationType over the ranges to which it should be applied.
Consider the AdornmentsHighlightWord sample's WordHighlightTagger. This effectively shows how to do it. Just change the style registered in the static constructor to this:
var style = new HighlightingStyle(null, brush);
style.Bold = true;
style.FontFamilyName = "Calibri";
Then you are good to go. If you have a definite list of ranges that should be highlighted this way, you can use a tagger that inherits CollectionTagger instead.
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