>I'm not really looking for a DIY type solution, here- more a
>product recommendation in the $50US range.  I don't mind
>working something up to provide the 30-second hold period, but
>I don't want to kludge up the locking mechanism.

How about one of those magnetic latches used for card entry door systems, 
where there is a steel plate on the door, and a whacking great electromagnet 
mounted on the frame? Done right the magnet could latch onto the back of the 
rack unit (if you used a steel back panel rather than aluminium) to stop it 
being pulled out. 

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