I'll point out first that I last touched this concept almost 3 years ago, so my memory may be fuzzy.
BUT, in BtS I got Firaxis to add my code so that MHP could be converted to a national wonder and only allow that Civ to make nukes. Hence why bAllowNukes only exists in BtS Civ4BuildingInfos.xml. Due to texts already being locked down I couldn't get a custom text added, so used the default.
So it will display "Allows nukes for all" but in reality only allows nukes for that Civ. So you must build the national project first, before you can build nukes.
OK?
I gave the funtion to a simple wall, and it allowed nukes for me too, when the enemy city had build them.
Oh and I uploaded my mod. It also has an City-destroing Nuke, as it was requested, by some users,