It was displayed on
http://explorer.dot-bit.org/ before, but as it is not useful anymore (currently there is 0.01NMC destroyed for each name_firstupdate, but it'll be 0 in some months) I've removed it.
Here is the info :
Total lost coins : 85127.53
To do the math yourself, you need each name_firstupdate tx with all tx_out and look at the tx_out with the script OP_RETURN. Not something you can do with the standard client.