@Eric:
I understand your concern about having only two people who are able to help obtain the Tenseigan, but I counter your concern with this:
How is it any different than someone seeking another out for training on a specific technique? Sage mode? IC knowledge of current events?
Answer is simple: It is not different.
Their pathway to obtaining that power creates RP. Would you honestly want people to run around saying they claim it without actually working for it? This way, it truly ensures we (as a society) will have documentation of whom achieved this particular ability.
I believe the Rinnegan should have been handled in this very similar way but it was too late to implement such a tactic. Especially with how (loosely stating) vague it was to obtain it.
Obtaining the Tenseigan is very specific. They found a way to bring it to SL. They achieved it via RP and as one of the watchers of the event, obtained it legitimately and pleasantly.
Why should they be punished (or in this case yelled at) for creating a path for more RP?
And should you get a challenger that claims the power of the Tenseigan, you will have the ability to address Warren and/or Rusaku to confirm their claim to be valid is true. If it isn't true, then you have a legitimate reason to revoke that specific ability for that challenge.
I did not at any point go caps out yelling, but that is besides the point. Secret techniques are not generally KG. In-game knowledge is not a KG in the strictest sense. The only way the two would be comparable is if some secret technique required to turn on Tensaigan were being guarded, then I can see how it is comparable. Even then, Tensaigan would transcend the ordinary technique guard precedents and would fall more into the forbidden/restricted jutsu (Edo Tensei, Hiraishin), making it the first dojutsu (I.E, the first of its kind) to fall under that category if memory serves. I didn't realize dojutsu power-ups could fall in that category.
Sage mode can be acquired just by in-game resets alone in some circles, so definitely incomparable in this situation. IC acquisition of IC knowledge of current events can be BS'ed so well for the sake of plot convenience that it is practically common practice to do so unless it involves an unwilling party, in which case proof of such IC acquisition tends to cut down on the IC knowledge acquisition hack.
So no, the answer is not "simply" no to that question. Regarding the challenge list, I would still opt to void it even if they had gotten Tensaigan through Warren and Rusaku.
... Who says we have to follow in the flawed footsteps of the Rinnegan, and accept the crap people want to claim? Me and Warren are put forth effort to create a system where the Tenseigan cannot be abused by anyone who just buys a 16 reborn and wants to get a chakra cloak.
This still does not seem to explain to me why you two and you two alone should be in charge of declaring who is worthy of receiving it IC and who is not worthy of receiving it IC. As Teostra suggested, while you two might could be subdued and forced into going through the ropes, I find it unlikely that either of your characters could be coerced just short of Koto to do such a thing, not to mention the difficulty in finding out that you even have the ability to do all of this.