That's an interesting suggestion, but one that can't be implemented the way you want I'm afraid :(
The canned replies are implemented as an editor-xtd plugin for Joomla since this is the only way Joomla's WYSIWYG editor will allow us to insert content to it. These plugins are called from the context of the editor which can not provide a reference to the ticket or category. This means we can't display per-category canned replies.
We can't add user group filtering; that's not how Joomla works. Joomla uses view access levels for displaying things and user groups for action privileges. Since we're talking about displaying that would be a view access level to each canned reply. Still, if a member of the support staff would satisfy the requirements for multiple view levels they'd see all canned replies for all view levels. Would that make sense in your use case?
Nicholas K. Dionysopoulos
Lead Developer and Director
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