To the best of my knowledge, no one has ever installed a billet-style grill on a NSX!
(thank god)
As far as fabricating some aluminum or resin mesh (basically gutter-guard or lathing sheet), many have done that for aesthetics as well as protecting the radiator and the A/C condensers. There are numerous DIY tutorials on the forum, just use the search function to find the relevant threads.
Also, there is a vendor here who offers some pre-fab' stone-shields:
Stone'd Shields: A/C Condenser Screen...
http://www.daliracing.com/v666-5/catalog/index_browse_part.cfm?focus=659
Stone'd Shields Radiator Screens...
http://www.daliracing.com/v666-5/catalog/index_browse_part.cfm?focus=668
If you decide to order, just make sure to get confirmation that they are in-stock; back-orders can be frustrating. . .