All right, then.
Chakra is created in the stomach, and Zanji's curse mark is on his chest, so it's very crucial we succeed, otherwise.... I'd prepare for the worst; either death, or a fate worse than such... being totally consumed by dark chakra...
Simply sealing the curse mark in this case would not be enough, so we're going to try to go further.
The curse mark creates yin chakra (evil chakra) and offsets the body's natural balance of yin chakra and yang chakra. If we simply seal it up, the yin chakra could easily seap into his stomach and contaminate the rest of the chakra, it's unique in this sense because of its close position to the stomach.
However, if the curse mark eminated yang chakra (good chakra) there would be no danger, and it might actually be a good thing.
We're going to have some extremely risky and life-threatening surgery
First, I'm going to further seal the curse mark, stopping all chakra flow throughout his chest, including his stomach. At this point, Zanji will have no chakra flowing through his body; that's where you come in, Mimi. Using your mystical palm technique, you'll put continuously cycle chakra into him, barely keeping him alive.
We'll have to work fast, before permanent damage sets in. At this point, UchlhaSasuke will use his Mirror Heaven and Earth change (or something) in combination with a Four Symbol Seal of mine. This will create a seal that gives Zanji access to the chakra of the curse mark, while using Zanji's own chakra to filter the yin chakra to yang chakra.
At that point, I will remove the five elements seal, reactivating his chakra flow.
After this is all over, he'll have to rest for a while before his chakra system recooperates. Afterwards, however, he should have access to extra yang chakra as Naruto does. However, Zanji can't overdo it, or he may become consumed by the curse mark chakra and, despite this now being yang chakra, that is not good.
Much like Naruto going 4-tails.