I hear ya on wanting more protection for your bearings.
My thought would be if you could fit an extra layer over your B.B.'s , you would still have the chance for grit to find It's way in past the seal on the back of the hub.

Keeping stuff out of your hubs is always worth it though, nothin sucks more then beating apart heat welded hub parts!!

 

I was in the same boat. (no pun intended). Mine had the typical plug style cap that went into the grease access hole. I then added these

http://images.oreillyauto.com/parts/img/large/bbi/rep-bearingbuddycover.jpg

 

I cut the top off a beer can and hose clamp it to the hub ( very fashionable!)