I would think this is not outside of the capabilities of Nagios. Otherwise you could periodically log in to the server remotely and check.
Hope this helps!
There might be a software or program that could perform the defragmentation process like controlling the remote computer as long as you have the access on the client you want to be defragged. However, it would be better if you perform the defragmentation procedure to the client computer itself rather that remotely doing it.