The code posted by Pageski is what i found. It has to be this that is file. rfe files are the data for arranging menus. So it's nothing to do with that.
The odd thing is that it appears to be already assigned. If you look at dc.english in hex, there is no reason why the Lions Tour text should not appear, as if the code is correct for the working text/modes before and after it, it must be correct.
I have a sneaky suspicion that you can get the modes to run, but only if you replace existing ones, or use existing paths. In theory, if you change the Lions Tour to the same code assignment as any other working mode, let's say tri-nations, lions tour now has a definite path, albeit a duplication of one and it will just copy tri-nations data, because that is what it is being told to load.
If you change the coding of the lions tour and 10 nations to a working modes coding (lets say tri nations) it should load the text for tri-nations in the menu space (stringID)where the lions tour text would be, and it should make a duplicate of the tri-nations. I can't say you will get a menu logo for it though, as the file for that clearly has 6 only.
You can then try and code the original tri-nations code/path with the coding for lions tour. It sounds confusing, but i think it may work like this.
See, all the text is there in the dc.english and will load if we can make a path for it i am sure. Rather like you can use a transparent graphic to hide a graphic, i noticed that the 6 nations and tri-nations are in the wrong order text wise to how they are in the game, which has prompted me to think that they have been shifted around to make the Lions Tour and 10 Nations unselectable. The easiest and laziest way to hide something is from it's start point. If they just make the text disappear, the game will still load the files, but of course you can't access them because they are invisible.
EA/HB won't remove anything from the data.gob, instead they leave it in so the foundation is left for future releases. Lions Tour and 10 Nations IS in the game, it is just lost, if you like and it think it is there.. rather like i can switch off a stadium with xml with a yes-no/true-false command, i think although there is no such file for modes, i think that if it's in data.gob it must be loading, it's just we can't see it because of some manipulative masking/coding.
I will continue to test.