filed against value of WI in mail template
2 answers
you could try : ${workItem.categoryName}
Not sure if it covers the path that you are looking for.. but the complete set of built in attributes that are accessible through the workItem variable type is listed in the link referred below.
So, if you haven't found the complete information, it may have to raise an RFE.
More samples for mail templates are available at :
Rational Collaborative Lifecycle Management Solution 5.0.2>Rational Team Concert>Reference>Work item Email notification template syntax>Email template variable types
hope it helps.