A couple of comments:
1. It would be good to create a separate VS for the port 53 traffic. You can then set a much lower idle timeout to clear out the connections quicker. We discussed a number of methods here:
http://devcentral.f5.com/Forums/tabid/1082223/asg/52/showtab/groupforums/aff/31/aft/1168069/afv/topic/Default.aspx
I think there is also an AskF5 solution on this, but I wasn't able to find it immediately. I'll post back if I do see it.
2. The DNS_* events are specific to GTM. There isn't an event that's triggered within LTM which directly corresponds to this. I'd try optimizing the virtual server configuration before worrying too much about which clients are generating the connections.
Aaron