Try a search for either something like a trailer tie down ring or trailer tie down d-ring. We used to buy them for various projects and either bolt or weld them down depending on the application.
Something like this?
http://www.etrailer.com/pc-TIE~LRB1.htm
Although this design will not allow you to Ubolt around the tongue, but your local weld shop could burn it on fairly easily.
You might even have some luck at a 4x4 parts warehouse, a bolt-on recovery eye.