In order to do this, you will have to customize the addcomment link. If you have one template for all, you need to detect switch the kind of link being generated, if you have a dedicated template for Service Desk, its not to hard,
instead of a http: link to the site, you would use a HREFof "mailto:reply@blah.com?subject=blah (ABC-123)",
Part 1: a UserMacro
#macro (showmailtocomment)
<table id="actions-pattern" cellspacing="0" cellpadding="0" border="0" width="100%">
#set ($subjectText = $textUtils.htmlEncode("($issue.getKey()) reply") )
<tr>
<td id="actions-pattern-container" valign="middle">
<table align="left">
<tr>
<td class="actions-pattern-action-icon-container">
<a href="mailto:reply@blah.com?subject=$subjectText" target="_blank" title="Comment"></a>
<img class="actions-pattern-action-icon-image" src="$attachmentsManager.getImageUrl("/images/mail/comment-icon.png")" alt="Comment" title="Comment" height="16" width="16" border="0" />
</a>
</td>
<td class="actions-pattern-action-text-container">
<a href="mailto:reply@blah.com?subject=$subjectText" target="_blank" title="Comment">Comment</a>
</td>
</tr>
</table>
</td>
</tr>
</table>
#end
Part 2:Customize the TemplateSet
Instead of:
#set ($commentActionBody="#parse('templates/email/html/includes/patterns/comment-action.vm')")
#rowWrapperNormal($commentActionBody)
Use:
#set ($commentActionBody="#showmailtocomment()") #rowWrapperNormal($commentActionBody)
That's the basic how to supply a reply to and a subject. determining when to use it in place over the standard is up to you in that context.