There are many examples of Lasombra antitribu. But are there Tzimisce antitribu? I am interested only in canonical characters, preferably those with character sheets. I know that it is mentioned as possible (even though very rare), but I do not know any examples.
The best answer is this:
There are no Tzimisce antitribu.
The vast majority of the Tzimisce belong squarely to the Sabbat; the rest are apolitical. There aren't even a bare handful of Tzimisce in the Camarilla, and those few are only there for personal reasons, and tend to leave once their objectives are completed. The presence of the Tremere virtually guarantees that the Tzimisce have no interest in remaining. Although a scant few individual members may join temporarily, it cannot be said that there is enough of an ideological difference, or enough members to declare an antitribu bloodline.
This is from unofficial white wolf wiki, but official book "Guide to Camarilla" from revised edition says exactly the same on page 53:
If there are a half-dozen Tzimisce in the Camarilla, it would be a major surprise. The vast majority of the clan belongs to the Sabbat; the rest are apolitical in the extreme. The very presence of the Tremere in the sect guarantees that the Tzimisce have no interest in signing on. Those very, very few Tzimisce who have deigned to join forces with the Camarilla have done so for intensely personal reasons (say, to avenge mortal insult or to use the Camarilla to dispose of a rival in the Sabbat. However, even under these dire circumstances the Camarilla`s few Tzimisce do not advertise their presence, do not take leadership roles in the sect and do not tend to stick around once their personal objectives are accomplished.
When you are speaking of "Tzimisce antitribu", you mean "Camarilla". No, it can only be a super rare temporary alliance. And the Old Clan has no connections to Camarilla. Check Hedeon Yaroslavich, for example. Of cause, the golden rule of storytelling applies here... But just don't do it. There are many interesting ways and stories, much better then "let's imagine Tzimisce antitribu".
And - nope, I haven't seen any such canonical characters.
The closest I can think of in regards to this, with characters having been canonized from this game and appearing in other materials, is potentially the "Sheriff" of LaCroix, who appeared in the "Bloodlines" game.
His clan is never explicitly given, but the fact he's said to have originated from Africa, along with his abilities would absolutely suggest him to be a Nagloper - which is a legacy bloodline of the Tzmisce.
But: while he works with a Camarilla Prince, it must be said that the circumstances of that alliance are unknown and suspect, especially with Sebastian's methods of getting (re: forcing) people to help him - and it's also clearly specifically to LaCroix and not the Camarilla as a whole, as the final fight against "Suckhead" went to show that he is more than willing to disregard any Camarilla rules (such as blatantly violate the Masquerade) in order to try protect the Prince.
Which goes along with the other answers that have been given to this question, in that: yes, as far as what canon has shown so far, if any Tzmisce were to work with any Camarilla it would be down to very specific circumstances and not due to any larger loyalty to the faction as a whole.