Tim Harper,
You can implement this request by this way :
Open the SupportClient.aspx , you can find :
<select id="input_department" style="width:220px;">
<%foreach(string department in ChatWebUtility.GetDepartments())%>
<%{%>
<option value="<%=Server.HtmlEncode(department)%>" selected><%=Server.HtmlEncode(department)%></option>
<%}%>
</select>
You can modify the list by your code , to limit the items to the departments as your want.
(for example , you can save the value to cookie before the user click the button , and pass the cookie to that list)
Regards,
Terry