Thanks for the tip. This sounds like it will work, but I'm having trouble.
I went to a task, opened the inspector and when to custom data. I added a Key called "Trac" and a url for the value.
I then edited the TaskTemplate.html file to look like this:
<h1>Task Report: {@Project Title@}</h1>
<table class="basic" id="task-report">
<tr>
<td class="header">ID</td>
<td class="header">Task</td>
<td class="header">Trac</td>
[SNIP]
{@Tasks@}
<tr class="{@Task Type@}">
<td>{@ID@}</td>
<td class="wrap">{@Title@}</td>
<td class="wrap"><a href ={@Trac@}>Trac</a></td>
<td>{@Start@}</td>
<td>{@End@}</td>
[SNIP]
On the Task Report page of the HTML Export, I now have a Trac column with a Trac link, but the value of that link is file:///Users/adhamh/Desktop/myproject.htmld/myproject.html insted of the http URL I entered as the value.
What did I do wrong?